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REESE LIBRARY

UNIVERSITY OF CALIFORNIA.

Received _^i4<^. . , .iS8^..

Accessions No. _ /'A? &A. . Shelf No..^_

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I

ENGMNEBRINO GEOLOGY.

IPs th^ 0amje 3^vdkox*

FIELD GEOLOGY, a Practical Guide in Geological
Surveying, with a Section on Falseontology, by A. J.
Jukes-Bkowne, B.A., F.G.S. Second Edition,
with illustrations and coloured plate. Croton 9/oo.

'We can confidently recommend the guide before us.' — Geological
Magcuine.

* We gladly hail the appearance of the present work, as satisfying a
want wnich has long been felt and frequently expressed.' — Nature.

Baillii^re, Tindall & Cox, 20, King William Street, Strand.

- N

LIBRAKY

UNIVERSITY OP

CALIFORNIA.

BNGINEEEING GEOLOGY.

W. HENRY PENNING, F.G.S.,

'» COLOOaXD PLATES.

LONDON :

BAILLlfeRE, TINDALL, AND COX,

20, KING WILLIAM STREET. STRAND.

1880.
[All Jtigkts Bewrved.1

Q E 33

EAfTH

SCIENCES

LIBRARY

/J'2> i^e)

1\^

.>Kj

AS A SLIGHT TRIBUTE OF ESTEEM

THIS WORK

IS INSCRIBED WITH THE NAME OF

CHARLES HUTTON GREGORY, CM.G^

{Paxi Pntident of the InstituHon qf Civil Engineen\

UNDER WHOM THE AUTHOR WAS PRIVILEGED TO

OBTAIN HIS EARLIER EXPERIENCES

IN ENGINEERING.

PREFACE.

This work first appeared, during 1879, in the pages of
The Engineer as a series of articles upon Engineer-
ing Geology, which, are now reproduced — slightly
altered in form, considerably enlarged, and more fully
illustrated.

Engineering and Geology are so evidently and so
intimately related that a knowledge of the former
must include, and is incomplete without, an acquaint-
ance with the latter; in turn. Geology derives much
aid from engineering works, records and researches.

It is as an art that Geology must be treated for its
results to be of immediate practical value to engineers ;
but as all art is based upon definite laws or principles,
he wiU derive most benefit from Geology, and be the
most proficient in its practical application, who founds
his work upon it, also, as a science.

W. HENRY PENNING.

Granville House,

FiNSBURY Park, London.

January, 1880.

CONTENTS.

PART I.

GEOLOGICAL STRATA, THEIR NATURE AND RELATIONS, AND
THEIR BEARING UPON PRACTICAL WORKS.

CHAPTER I.

PAGE

Introduction - - - - - - 1

CHAPTER 11.

GEOLOGICAL STRATA.

Geological Strata.-— Nature of the Rocks. — Relations of
the Rocks. — Bearing of the nature of the Rocks upon
practical works — Railways — Tunnels — Embankments —
Bridges ------- 8

CHAPTER III.

GEOLOGICAL STRATA {continued).

Bearing of the nature of the Rocks upon practical works,
continued — Matei'ials — Minei^als — Metals — AgricvUure
— Land-drainage —Sewerage works - - - 28

CHAPTER IV.

GEOLOGICAL STRATA {continued).

Bearing of the relations of the Rocks upon practical works
— Mining operations — Railway cuttings — Tunnels — Em"
bankments — Reservoirs — Canals — Main -drainage —
Foundations — Water-supply — Dampness — Disease - 41

CONTENTS.

PAET II.

PBOCEDUEE IN THE FIELD.

CHAPTER I.

METHODS EMPLOYED IN GEOLOGICAL SURVEYING, MAPS

AND SECTIONS.

PAGE

Methods employed in geological surveying — Det^^nation
ofrock9 — Tables - - - - - - 54

CHAPTER 11.

METHODS EMPLOYED IN GEOLOGICAL SURVEYING

{continued).

Construction of geological maps — Geological surveying —
Example— Dip - - - - - - 03

CHAPTER III.

METHODS EMPLOYED IN GEOLOGICAL SURVEYING

(continued).
Dip and strike — Rules for finding dip — Example - - 72

CHAPTER IV.

METHODS EMPLOYED IN GEOLOGICAL SURVEYING

{continued).
Example of surveying faulted area — Drift deposits - 78

CHAPTER V.

METHODS EMPLOYED IN GEOLOGICAL SURVEYING

{continued).
Geological sections — Practical value — Levels -Filling in - 92

CONTENTS. XI

PAET III.

ECONOMICS — MATERIALS, MINERALS AND METALS;
SPRINGS AND WATER-SUPPLY.

CHAPTER I.

MATERIALS, MINERALS AND METALS.

Economic products of the Recent and Tertiary Rocks - 99

PAGE

CHAPTER II.

MATERIALS, MINERALS A1H1> METALS (continued).

Economic products of the Secondary Rocks - - 110

CHAPTER III.

MATERIALS, MINERALS AND METALS (corUinued).

Economic products of the Palaeozoic Rocks - - 125

CHAPTER IV.

SPRINGS AND WATER-SUPPLY.

Nature of Springs— /S^wz/ace Springs — Deep-secUed springs
—Water-level 131

CHAPTER V.

SPRINGS AND WATER-SUPPLY (continued).

Artesian Wells — Absorption Wells - - - - 145

CHAPTER VI.
Building Sites - - - - - - 154

LIST OF ILLUSTKATIONS.

PAGE

Plate I. Geological Map — coloured - Frontispiece

Figure 1. Eailway cutting in pervious and impervious

strata - - - - - - 45

„ 2. Railway tunnel through a ridge - - - 47

„ 3. Diagram to illustrate Rule for finding true dip - 75
„ 4 Map of area geologically surveyed - - 80

„ 5. Section of strata - - - - - 87

„ 6. „ across area geologically surveyed - 97

„ 7. „ to illustrate the nature of springs and

their water-level . - - - 132

Plate II. Geological Section — coloured- - to face 140

ENGINEERING GEOLOGY.

PART L

GEOLOGICAL STRATA, THEIR NATURE AND RELATIONS,
AND THEIR BEARING UPON PRACTICAL WORKS.

CHAPTER I.

INTRODUCTION.

In the exeoution of engineering works, however scien-
y tific in design and clever in workmanship, failure has
(' frequently usurped the place of success, because due
attention has not been paid to geological phenomena.
Numberless instances might be quoted in proof of this
proposition, whilst it is notorious that vast sums of
money have been thrown away in mining speculations
which would at once have been characterised as hope-
less by anyone possessing the slightest acquaintance
with the science of Greology. A late eminent authority
(Professor Jukes) has stated his belief that the amount
of inoney fruitlessly expended in a ridiculous search
a.fter coal, even within his own experience, would have
paid the entire cost of the Government Geological
Survey of the United Kingdom.

Although a knowledge of this science is undoubtedly
a great acquisition, which afEords both pleasure and

ENGINEERING GEOLOGY.

profit to its possessor, it is not possible, nor even de-
sirable, for all professional engineers to become pro-
ficient geologists. Those for whom this work is more
especially intended have too many claims npon their
time and attention to bestow either upon a study of
abstract principles, laws, and theories, which do not
relate to their own particular science, art, or occupa-
tion ;, but they may nevertheless, and with advantage,
avail themselves of the labours of others, when the
results of those labours bear directly, and in a very
important degree, upon the stability or success of the
works designed or executed by them, or under their
superintendence.

The engineer should certainly make himself ac-
quainted with the geology of a district through which
a railway is to be constructed from his designs and
along the line of his selection. He should ascertain
the nature of the various rocks that will be met with,
not only at and near the surface of the ground, but for
a considerable distance below; their relation to each
other, and the important influence they will exert upon
the works in contemplation. • Trial-holes * are generally
dug for this purpose, but these are simply pits exca-
vated to a depth of a few feet, and afford information
which, although valuable in itself, unless amplified in
a partioulai* manner, extends only to the superficial
deposits. Deep borings are sometimes made, but are^
in most cases, too costly ; and however numerous these,
or trial-holes, may be, both foil far short of what can
be achieved in the same direction through the methods
employed by the field-geologist. By these are deter-
minedj not only the kind of rocks occurring at or near

INTRODUCTION.

the surface, but also their position in regard to each
other ; and the geological surveyor is enabled fco indi-
cate with reasonable accuracy what strata will be met
with to a depth, it may be, of several hundred feet,
and, what is of equal importance, the order of their
succession. These results of his labours include not
merely a knowledge of what beds would be pierced in
sinking a well, or in excavating trenches for founda-
tions, such as would be afforded equally by trial-holes
or borings of sufficient depth ; but they embrace also
the important points of the ' lie ' of the beds, the order
of their superposition, their outcrop, dip, and conse-
quent water-bearing properties ; by all of which the
stability and durability of engineering works are greatly
affected.

We cannot fail to perceive how differently placed or
constructed would have been many of the most im-
portant works, such as fortifications, railway-cuttings,
embankments, tunnels, and even sewers, had those who
designed them been acquainted with the principles,
methods, and results of field-geology; or how much
capital might have been usefully instead of fruitlessly
expended, or how many catastrophes would have been
averted. Mention has already been made of costly
sinkings for minerals, where they could not possibly
have been found ; large sums of money have also been
wasted in equally fruitless searches for water. Yet
water-supply is as amenable to known laws as any*
other phenomenon of nature, and within certain limits
it may be determined without experiment. Although
the divining-rod has not even yet quite ceased to be a
power amongst us, its days are surely numbered ; men

1—2

ENGINEERING GEOLOGY.

must, sooner or later, come to see that springs are
merely the result of water finding its own level, and
that for water to issue forth at one part of the earth^s
surface, it must have been absorbed at another. When
the conditions affecting its absorption by and passage
through the strata of a district are known or can be
discovered, the existence of springs, their depth from
the surface, and the height to which they will ascend,
can be approximately, if not with extreme accuracy,
determined.

In his ^ Rudimentary Geology,^ Major-General
Portlock has truly and eloquently said : ' Geology
is now a true science, being founded on facts and
reduced to the dominion of definite laws, and in
consequence has become a sure guide to the practical
man. The miner finds in it a torch to guide him in
his subteri'anean passage, to the stratum where he
may expect to find coal or iron, or to the recovery of
the mineral vein which he has suddenly lost; the
engineer is guided by it in tracing out his roads or
canals, as it tells him at once the firmest stratum for
supporting the one, and the easiest to cut through for
the other, and makes him acquainted with the qualities
of the materials he should use in his constructions,
and the localities where he should seek them; the
geographer finds his inquiries facilitated by learning
from geology the influence of the mineral masses on
the form and magnitude of the mountains and valleys,
and on the course of rivers ; the agriculturist is taught
the influence of the mineral strata on vegetable and
animal life, and the statesman discovers in the effects
of that influence ^a force which stimulates or retards

INTRODUCTION.

population; the soldier also may find in geology a
most valuable guide in tracing his lines both of attack
and defence ; and it is thus that a science rich in the
highest objects of philosophical research is at the same
time capable of the widest and most practical applica-
tion/

In the following pages, rules and methods relating
to stratigraphical geology only are given, as the
geological conditions which affect engineering and
similar works are, mainly, the extent of the various
strata, their lithological character, and their order of
succession. It matters not what may have been the
forms of Life during the ages when the strata were
deposited, what their relations to those older or more
recent, or what the order of their appearance in time ;
although the evidence regarding these points is as
strong and as interesting as any npon which is based
the science of Geology. The rocks are treated merely
as stones, clays, and sands of varying kinds; some
possessing commercial value and great utility ; others
having qualities to be guarded against in all mechani-
cal operations; some only exhibiting water-bearing
properties ; but all worthy of study, independently of
the old-world histories which they contain.

The names of places are given only in particular
instances, such as those of mines, important quarries,
notable sections, and so on, it having been considered
advisable not otherwise to refer to localities in the
description of the rocks. These are mentioned
generally, and under specific denominations, geological
maps indicating much more readily the formation at
any particular spot than a lengthy reference to the

ENGINEERING GEOLOGY.

many places which must otherwise have been men-
tioned as situated on an extended outcrop. Such
maps are generally too small for the boundaries of
formations to be defined upon them with extreme
accuracy; indeed, they are not intended for that
purpose, but rather to indicate what set, or sets, of
beds an observer may expect to find in any particular
neighbourhood. When an accurate delineation is
required for any special purpose it may be found
upon the sheets (corresponding in size and index
numbers with the Ordnance maps) published by the
Government Geological Survey. These are laid down
upon a scale of one inch to a mile for the country
generally, and for some districts upon the large scale
of six inches to a mile ; their prices vary, of the former
(some of which may be obtained in quarter-sheets)
from 4s. to 8s. 6d. ; of the latter from 4s. to 6s. (p. 17).
The main object of this work is, however, to enable
the engineer to discover and, for all practical purposes,
to trace out for himself the nature and extent of the
rocks with which he is concerned.

An acquaintance with the methods of geological
surveying is the more valuable, because ' drifts^ are
usually omitted from the maps ; these are a series of
superficial deposits which, although important in some
localities, are not shown on any of the older geological
charts, and are noticed on only a few of the more
recent official publications. They consist chiefly of
clays and gravels of peculiar character, which are
found here and there upon the older rocks, on hills
and in valleys, with no very definite mode of occur-
rence j and although, as a rule, of no great thickness.

INTRODUCTION.

they of necessity exert a considerable influence over
all works constructed upon them. A section is
devoted to a brief description of these deposits, with
the methods of tracing and mapping them, as they
must also be treated from a practical point of view, in
the same way as the older and more generally impor-
tant formations.

'/^-

LIBRA K 1

UNiyEl?SITY OF

CALIFORNIA.

CHAPTER II.

GEOLOGIOAL STJSATA.

Geological Strata. — ^Nature of the Eocks. — Relations of the
Rocks. — Bearing of the nature of the Rocks upon practical
works— BaUwaya — Tunnels — Ernbankments— Bridges,

Geological Strata, — The crust of the earth consists of
a great number of alternating rooky layers, various in
kind^ thickness, and extent, but always in regular, if
not in constant, sequence. The uppermost have been
formed in a great measure from the waste of those
beneath, in the same way as the material now being
deposited on the bottom of the sea has been derived
from the denudation of the present dry land. These
layers are but rarely horizontal, and they bear evidence
of having been subjected to some upheaving force
which has acted at various times, unequally and with
different degrees of intensity, beneath every portion of
the earth's surface. There have been corresponding,
and on the whole nearly equal, movements of depres-
sion, and all areas have frequently been dry land>
again to be covered by the waters of the ocean. It
is owing to this inequality in the upheaval of the
beds, and to their consequent partial destruction by

GEOLOGICAX STRATA. ^

the sea, that the lower and older strata are now
exposed at the surface of the ground, and that we
are enabled to classify the rocks and to decipher their
ancient history.

The formations, of which the denuded edges are thus
bared and thrown open to our inspection, are ihdicated
by different tints upon geological maps. If it be borne
in mind that each of the areas thus distinguished re-
presents, as a general rule, the edge and not the surface
of a formation, the proper apprehension of such maps
is much facilitated. It is evident that were the
variously-coloured portions each indicative of an origi-
nal surface-plane, the rocks so depicted would generally
be the newest, as overlying those which are hidden
beneath. But their edges only being exposed and
portrayed on the map, the main planes of bedding must
now be either in a vertical position, or inclined from

the surface in some direction, and the rocks, as a
matter of course, must pass in under some of those
that are contiguous. Geological maps show that, in
this country, by far the larger proportion of the edges,
or lines of outcrop, of the rocks, follow a nearly north
and south direction, therefore the beds must dip, if at
all, either to the east or to the west. The general dip
of the rocks in these islands is, on the whole, towards
the south-east; consequently those on the north-west
are the oldest, and the lowest in the geological scale ;
those on the south-east are the highest in the scale,
therefore the most modem.

All the beds of which the various geological forma-
tions are composed are termed * rocks,' whether they
are hard or soft, of aqueous or of igneous origin. The

10 ENGINEERING GEOLOGY.

following remarks have been as far as practicable
classified under three headings — (a) The natare of the
rocks; (b) The relation of the rocks to each other;
(c) The bearing of the nature and relation of the rocks
upon practical operations.

(a) The Nature of the Bocks. — The aqueous and
igneous deposits by which the known crust of the
earth has been built up, occur in successive layers, and
are of infinite variety as regards texture, colour, hard-
ness, and other peculiarities. All the rocks are made
up, wholly or in part, of minerals either in a crystalline
or fragmentary form, or of mineral matter in a state of
comminution. Some rocks contain metals, either in a
free or native state, or, as ores, in combination with
oxygen or sulphuric acid, whilst comparatively few are
without metallic colouration.

All rocks may be divided into two great classes :— :

1. The igneous, or unstratified, which (formed below
the surface) were by volcanic or some similar force
erupted through or intruded into the pre-existing
formations. These are granites, traps and similar
rocks.

2. The aqueous, or stratified, which were deposited
from water as sediment, or (in some cases) as a
chemical precipitate. They are chiefly clays, sand«
stones, limestones, and gravels.

There are rocks which have been otherwise formed,
and some which have been altered from their original
condition by heat or pressure, or by both agencies
combined. Such metamorphic rocks may have been
either igneous or aqueous, but are principally of aque*

GEOLOGICAL STRATA. 11

ous origin, and are now found as gneiss^ quartzitesj
marbles, slates, schists, and altered ashes.

The class to which a rock belongs is practically im-
portant, on account of the difference in the normal
modes of its occurrence. The stratified rocks lie evenly,
the one upon the other, whether horizontally or not,
and preserve a regular but sometimes interrupted
sequence j the unstratified follow no such definite lines,
but are found suddenly breaking through older rocks
and disappearing in an equally abrupt manner. In
both classes the rocks of every kind present many
varieties and gradations towards each other, but on the
whole they possess broad characteristics by which they
may be fairly determined. (See Some chapters on
Lithology, and Tables for the determination of rocks,
in 'Field Geology^ (Bailli&re), from which those in
Part ii. have been abridged.)

It may be noted that generally, but not without
exceptions, the older stratified and the altered rocks
are more crystalline and compact than are those of
more recent date. Those that were by an old classifi-
cation designated Primary, consist of slaty and crys-
talline strata, such as gneiss, and mica-schist, marble,
and clay-slate ; Transition, of slaty and siliceous sand-
stones and calcareous shales; Secondary, of chalk,
limestones, red sandstones, marls, and clays ; Tertiary,
of sands and clays; Recent, of sands, gravels, i gilt,
peat, and alluvium. The loose and friable beds are
the most recent, overlying others more consolidated of
secondary age, which in turn rest upon the more
crystalline primary strata. All were once in the same
unsolidified condition, but some have become hardened

12 ENGINEERING GEOLOGY.

by chemical change and by the mechanical results of
pressure and infiltration^ during the ages which have
elapsed since the time of their accumulation.
. 1. The more common igneous rocks are : —

Granite. \ ^

Syenite. | Gramtic.

Gabbbo. — ' Greenstone.' ) rn

T^ > Trappean.

Fblsite. J ^^

DoLERiTB. — Basalt.

Volcanic.
Trachyte.

POEPHYRITE. /

Phonolite. r

2. The aqueous rocks are : —

Argillaceoiis.
Clay.

Shale. — Hardened clay.

Loam. — Clay and sand^ a mechanical admixture.

Limestone — when containing silicate of alumina: if

this be in sufficient proportion it constitutes an

hydraulic limestone.

Arenaceous,
Sand.

Sandstone. — Consolidated sand^ with siliceous, ferru-
ginous, or calcareous cementing material.
Grit. — Coarse sandstone.
Gravel.
Conglomerate. — Consolidated gravel.

GEOLOGICAL STRATA. 13

Calcareous.

Limestone. — Sometimes earthy as Chalk, oolitic as
Bath freestone, crystalline as Marble.

Magnesian Limestone. — Limestone composed of car-
bonate of lime and magnesia.

Siliceous Limestone. — Limestone containing much
silica, as in Kentish Bag.

The altered or metamorphic rocks are : —

Gneiss. — ^A foliated rock, otherwise similar to granite
in composition.

Schist. — Sedimentary rock, altered and foliated.

Quartzite. — Altered quartz sandstone.

Slate. — Clay altered and cleaved in a direction gene-
rally transverse to its original bedding.

There are many other rocks in each class, also
many which partake more or less of the character of
each, presenting infinite gradations ; but they occupy
comparatively small areas, and in other respects exert
the same influence as the rocks to which they are
most nearly allied, they may therefore, from an
engineering point of view, be considered as of slight
importance.

(6). The relation of the Bocks to each other. — The
relation of the rocks of a district — that is, their posi-
tion in regard to each oth^r, their relative thickness,
dip, permeability, and so on — ^is quite as important for
mechanical purposes as their individual nature. But
this relation, especially in a complicated area, is not
by any means readily ascertained, unless the proper

14 ENGINEERING GEOLOGY.

methods of procedure be understood. The thickness
of each necessarily rules the extent of ground it
occupies^ but must be studied in connection with the
dip^ which exercises also an influence on the shape of
the country quite as powerful as that of the nature of
the rocks of which it is composed.

Where the bedding of rocks is horizontal, or nearly
so, the surface will be much more flat and spreading
than where the dip is sharp, a condition which will
produce a rugged and rapidly- alternating landscape.
This fact is well worthy of notice, because we may
reason conversely that if a country be flat, the local
beds are tolerably level, and extend some distance in
any direction; but if it be much broken, that they
rapidly disappear, having a high angle of inclination.
Upon the dip other properties of the beds depend;
and it will be seen that it afEects both directly and
indirectly the works constructed on their outcrop. The
relative elevation of varying deposits bears directly
upon the flow of surface water from one area to
another ; therefore it affects the land springs, and in
the same degree the dryness or dampness of any given
locality. The question of relative permeability is more
extensive and intricate, and upon this depend the all-
important points of the power of absorption of water
by the beds, and the nature and origin of deep-seated
springs. These points influence not merely the auppljr
to Artesian wells, but the liability to landslips* and
must be considered also in calculating the varying
pressures by which engineering works are especially
affected.

The phenomena of deep-seated springs^ just referred

GfiOLOGIGAL STRATA. 15

to, depend not altogether upon permeability — although
this is one of the chief elements in their production —
but also upon the relative position of pervious and im-
pervious strata. These may succeed each other in the
simplest way, by being in regular sequence, with the
higher beds resting evenly upon the lower, each pos-
sessing the same angle of dip ; or in a more compli-
cated manner, which is described as ' unconformable/
This term is applied to beds, or to sets of beds, which
at any particular spot rest one upon the other, but
possess different degrees of inclination. It is evident
that in such a case the uppermost beds rest upon the
edges, and not upon the surface of those beneath,
and that before the higher were deposited, the lower
had been cut off by some process of denudation.
Occasionally beds overlap each other, without being
exactly unconformable; and sometimes those which
are known to be So, do nevertheless rest evenly upon
each other, and with the same dip ; but this is a
merely local, and may be considered an accidental,
occurrence.

When strata are unconformable, of course the con-
tinuity of the beds beneath the surface is broken, and
this must affect, more or less, the flow of water
through them in any direction. The underground
extension of rocks is likely to be interrupted also by
'faults,' or other dislocations, by which portions of
them are displaced. Sometimes several hundred feet, x" ^-^k,
and which may extend horizontally for a short distano^ -^
only, or for several miles. It is necessary th»f' ^rV" . ^
such faults or breaks be discovered, and , theifi^ik- * ^
fluence estimated, in the consideration ^^rcjt^ms in \ > . s
engineering geology. f X^ .V^^ o^>

16 ENGINEEBING GEOLOGY.

The relations of the rocks upon a grand scale^ that
is classified into 'formations' and 'series/ are shown
upon good geological maps with as much minuteness
as is possible upon the scale employed. The one-inch
maps made and published by H.M. Geological Survey
afford all needful information for general pui*poses,
whilst the six-inch maps admit of greater detail and
accuracy; but the former are not yet published for the
whole country, nor are the latter for more than a com-
paratively small, although an important, portion. And
there are frequent and sometimes rapid changes in
the nature of the beds, which cannot be shown on maps
that portray generally the structure of a district,
although the local beds may be, and usually are,
described in the memoirs by which many of the official
maps are accompanied.

These local peculiarities render an acquaintance with
the methods of geological surveying valuable to the
engineer, who would thereby be enabled to discover
and to trace out for himself all local changes and
intercalated deposits by which his works may b^
greatly affected. For instance, in many thick clay
formations, shown wholly as such on the maps, there
are limited beds of limestone, sometimes in considerable
number and of excellent quality, which may be utilised
for building, for lime, or for ballast; on the other
hand, there are in similar positions, intercalated
deposits of permeable material which yield water that,
in some cases, must be guarded against, in others,
may be turned to good account.

The following list of the maps officially published by
the Geological Survey may be found useful for

GEOLOGICAL STRATA. 17

reference, the numbers of the sheets being precisely
the same as those of the Ordnance Survey.

LIST OF MAPS,

PUBLISHED BY H.M. GEOLOGICAL SURVEY.

Scale, one inch to a mile.

England and Wales.

Large sheet, London and its Environs - 22

„ „ „ „ with Drift deposits 30
Sheets, representing an area of about 800 miles.

No. 1. In quarter sheets - each quarter 3

2. Whole sheet - - - - 4

3. ,,,,----86

4. „„...- 5

5 and 6. „ „ - - - each 8 6

7. „ „ 8«. 6c?. With Drift deposits 18 6

8 and 9. „ „ - - - each 8 6
10. » », - - - -40

11 to 22 » n - * - - each 8 6

23 and 24. « „ - - - » 4

25aw€?26. „ „ - - - » 8 6

27 to 29. „ „ - - - „ 4

30 awe? 31. „ „ - - - „ 8 6

32. „„ - - - - 4

33^0 37. ,, „ - - - „ 8 6

38awcZ39. „ „ - - - ,, 4

40 and 41. „ „ ' - - - „ 8 6

42 and 43. In quarter sheets - each quarter 3 O

44. Whole sheet - - - - 8 6

45 and 46. In quarter sheets - „ ,j 3
48. S.E. Quarter sheet, 3*. With Drift

deposits - - - -30

52 to 56. In quarter sheets - each quarter 3 O
57. N.E., S.W., and S.E. Quarter sheets

each 3

57. N.W. Quarter sheet - - - 1 6

ENGINEERING GEOLOGY.

No. 58.
59.

60 to 63.
64

71 to 75.
76.
76.
77.

78 to 82.
87 to 89.
90.
91.
92.

98.
101.
105.
109.

Whole sheet - - - -

N.E. and S.E. Quarter sheets, each

8. d.

In quarter sheets
Whole sheet -
In quarter sheets
N. Quarter sheet

S.
N.E.

- each quarter

})

»»

3
3

8
3
1
3
1

y In quarter sheets

^Quarter sheets,
each quarter

{

N.E. and S,K
N.W. and S.W.
aw. and S.E.
N.W. and S.W.
N.K, S.W. and S.E.
S.E.

In quarter sheets
S.E. Quarter sheet

Six-inch maps are published of parts of Northumberland,
Cumberland, Durham, Lancashire, and Yorkshire, in sheets
representing about 24 square miles, at 4$. or 65. per sheet.

»>

3
3

Scotland.

No. 1 and 2.
3, 7 and 9.
13.

14 and 15.
22, 23 and 24
32 and 33.
34.
40 and 41.

Whole sheets

)

99
99

each 4

-„ 6

-9, 6

-99 6

Six inch maps are published of parts of Edinburghshire,
Haddingtonshire, Fife, Ayrshire, Benfrewshire, Dumbarton-
shire, Lanarkshire, Dumfriesshire, Stirlingshire, Perthshire and
Clackmannanshire, in sheets representing about 24 square
miles, at As, or 6s, per sheet.

GEOLOGICAL STRATA.

TkeTiAND.

«. d.

No. 21, 28, 29, 36,

36 and 37.

Whole sheets

- i

°»ch 3

1 6

47, 48 and 49.

60.

1 6

53, 59 to 61, 66 to 71.

72.

1 6

74 to 81.

82.

1 6

83 to 121.

122.

1 6

123 to 130.

131.

1 6

132 to 134

136.

136 to 139.

140.

1 6

141 to 149.

150.

1 6

151 «o 158.

»9

159 and 160.

1 6

161 to 169.

170.

>»

1 6

171 to 179.

180 to 182.

1 6

183 to 188.

189 and 190.

1 6

191 to 195.

■•

196 awc^ 197.

«•

1 6

198 to 201.

202 to 205.

1 6

Six-inch maps are published of parts of the counties of Ros-
common, Leitrim, SHgo, Antrim and Tyrone, in sheets repre-
senting about 24 square miles, at 48, or 6s. per sheet

Geological Sections, on a natural scale of six inches to a
mile, traversing England and Wales, Scotland and Ireland, in

2—2

20 ENGINEERING GEOLOGY.

various directions, are also issued, mth descriptions, in sheets
representing about 36 lineal miles, numbered 1 to 121, at 6^.
per sheet.

There are also Vertical Sections, on a scale of 40 feet to an
inch, through Coal Measures and other important strata, in
sheets numbered 1 to 62, at 3.9. Qd, per sheet.

Memoirs on separate areas, and on various subjects in
Physical Science, and Eecords of the School of Mines and of
Science applied to the Arts, are also published by the Geo-
logical Survey. (For full particulars of all these works and
of the various maps and sections, see the ' Catalogue of the
Publications of the Geological Survey of the United King-
dom.')

(c) The Bearmg of the Nature (a) and Relation (b)
of the Rocks upon Practical Worles. — The nature of the
rocks which form the surface of any district is indicated
generally upon geological maps of the area in question,
and such maps are of great value, to the engineer, as
the rocks, whatever their nature may be in any parti-
cular locality, of course form the base of all engineer-
ing and also of all architectural works. The same
remark applies equally to mining and well-boring
operations, and to agricultural pursuits; the rocks
must, of necessity, exercise over them all a permanent
influence, greatly conducive either to their success or
to their failure. The rocks exert this influence not
only by their inherent properties or peculiarities, but
also, by the relations which they bear towards one
another. These relations are shown in the sections,
which amplify the information supplied by the maps ;
they have been briefly mentioned above, and are more
fully described further on. Some of the many instances
in which the influence of the rocks is exercised may be

GEOLOGICAL STRATA. 21

here enumerated^ as indicating the necessity for all
available geological information being secured, and
calculations based thereupon, before works of any
importance are commenced or even designed. This
influence is exerted in two distinct ways, (a) Through
the nature of the rocks ; (6) Through their relation ;
but the two, although distinct, are sometimes blended,
and always so intimately associated that a partial
separation only can be attempted.

{a) THE BEARING OF THE NATURE OF THE ROCKS UPON

PRACTICAL WORKS.

Cuttings. — The trial-holes .usually made before the
commencement of railways, and other engineering
works, enable the engineer to determine the kinds of
rock which come to the surface along a given line, or
over a given area. But these holes, by themselves,
afFord no indication of the thickness of any great
deposit, and in this respect may mislead, unless the
actual conditions be otherwise ascertained; a knowledge
of several particulars can, however, be derived from
trial-holes, when once the proper method of utilising
their indications is understood. For instance, a line
of such holes traversing a hill may be, almost without
exception, in clay, one or perhaps two of them on the
flank of the hill being sunk in a hard rock. The
inference would probably be that a cutting made
through this hill must pass entirely through clay, ex-
cept at those points where the hard rock was actually
exposed ; the reality, that nearly the whole of the work
has to be carried on through the harder stratum, per-
haps at a great additional cost. For the edge only of

22 ENGINEERING GEOLOGY.

the bed was tonclied by the one or two holes where it
comes to the snrface^ with a narrow outcrop^ along the
steepest part of the hill.

This sort of thing has repeatedly happened, and
more frequently still, a bed of gravel or sand, only a
few feet in thickness, but spread over an extended
area or on the slope of a hill, has led to the conclusion
that the whole of the cutting would be, as all the trial-
holes were, in sand or gravel. It may turn out to be
hard and intractable rock, removable only by blasting
operations, but covered with gravel just thick enough
to reach below the few feet exposed in the trial-holes.
Or the converse may be the case ; the engineer un-
acquainted with geological methods feels sure, from
his trial-holes, that he will obtain from a certain rail-
way cutting, building-stone sufficient for all the
bridges, or gravel enough to ballast the line ; but as
the work progresses, and the deeper strata come into
view, he meets with serious disappointment. These
instances, selected from many, are sufficient to show
the necessity for the exact nature of the rocks being
ascertained, not merely at the sur&ce, but to a depth
below, certainly not less than that of the deepest cut-
tings. (See also Banksy p. 25.)

The nature of the rocks to be passed through in
cuttings materially affects the preliminary calculations
in regard to the slopes and necessary widths, which
differ accordingly, for the ' wgles of repose ' vary
directly as the nature of the rocks in question. It
will be seen from the following table that these angles
cover a wide range, from the very flat slope of 14* up
to a vertical face, if the harder rocks not named in the
table be taken into consideration.

GEOLORICAL STRATA, 23

TABLE OP NATURAL SLOPES OP EARTHS,

WITH EEPEEENCB TO A HORIZONTAL LINE.

Moktworth.* San&ine.i'

14° to 17°

21° to 37'
35° to 48°

Clay, wet -

- 16°

Sand „ -

- 22° ^

Vegetable soil

- 28° [

Sand, dry -

. 38° )

Shingle

- 39° -j

Gravel

- 40° 3

Rubble -

- 45°

Clay, well drained -

- 45°

Compact earth

- 50°

Chalk, when compact, will stand in cuttings with a
vertical face, as will the tougher and more finely
grained varieties of sand when closely bedded ; indeed
a series of narrow benchings, with vertical faces, is
to be preferred tb a uniform slope in such material.
The slopes of fhe banks formed of the expavated
material will, of course^ be the same as those given
in the above tables.

The hard rocks, such as limestone and sandstone,
will stand safely in cuttings at angles varying accord-
ing to the number and extent of joint- and bedding-
planes ; when these are few and narrow the face may
be vertical, being sloped or benched back as they
become more numerous or extensive. In such cases
the dip of the beds must be considered; a &ce may

* * Pocket-book of Engiueering Formul».'
t ' Manual of Civil Engineeriug.*

24: ENGINEERING GEOLOGY.

he vertical on the lower side of a cutting as regards
the dip with perfect safety, but on the upper side may
have to be cut back very considerably, especially if
the material between the beds of rock be at all of a
yielding or slippery nature. (See also p. 45.)

Profesaor W. J. M* Eankine, in his * Manual of Civil
Engineering' (1864, p. 317), makes the following remarks on
this important subject : —

' When rock is firm and sound, so that the permanence of its
cohesion may be depended upon, the sides of excavations in iti
may be made vertical, or nearly so.

' How far the cohesion of the rock is to be depended upon,
is a question to be solved rather by observation of the rock in
each particular case, than by any geological principles having
regard to its geological position, mineralogical character, or
chemical composition ; for the geological position is fixed by
the organic remains imbedded in the rock ; and these have no
connection with its mechanical properties ; and rocks composed
of the same species of minerals, and the same chemical con-
stituents in the same or nearly the same proportions, show
great differences in strength and durability.

*It may be observed, however, that the cohesion of igneous
and metamorphic rocks, such as granite, syenite, trap, gneiss,
mica-slate, marble, quartz-rock, etc., may in general be trusted,
unless they are much fissured, or contain potash-felspar, in
which cases a sufficient slope must be given, to prevent frag-
ments from falling into the cutting. Of the sedimentary rocks,
those which contain much clay, such as shale, are to be treated
with caution, how hard soever they may be when first cut ; for
they are liable to soften by the action of the weather. Sand-
stone and limestone, whether compact or granular, if fit for
building purposes, will stand with vertical or nearly vertical
faces ; but those materials exist of every degree of hardness, from
that of rock, properly speaking, to that of earth. Sandstone
is met with which crumbles in the hand, and requires slopes of
from 1 to 1 to 1 1 to 1 ; and chalk, according to its degree of

GEOLOGICAL STRATA. 25

hardness and soundness, stands at slopes varying from ^ to 1
to IJ to 1.

* The stability of sedimentary rocks in the side of a cutting
is greater when the beds are horizontal, or dip away from the
catting, than when they dip towards it.'

Tunnels. — If information regarding the nature of
the rocks be necessary for cuttings through them, it
is much more required for tunnels, where the work is
far more costly, and where, consequently, much greater
saving may be effected by a previous knowledge of
what, strata will, or will not, be passed through in any
line at a given level. Yet here the indications from
trial-holes must be still more meagre, seeing that
tunnels are seldom made except where the hills are
too lofty to be passed through by open cuttings, con-
sequently through strata at a greater distance from the
surface. But the evidence so obtained, if treated by
geological methods, may be made equally reliable, and
in proportion, far more valuable. Many tunnels have
been made in places which would have been sedulously
avoided had the geological phenomena been previously
ascertained; others, where, by diverting the line a
short distance to one side or the other, the tunnels
might have been made with a saving of more than
half the cost of construction.

Banks, — The nature of the rocks to be passed through
in tunnels or cuttings affects also the calculations for
the necessary slopes, and consequent widths, of em-
bankments. For the ' angles of repose ' vary in this
case also, as the material, of which they are to be
formed, is sand, gravel, clay, chalk, or rubble; the

26 ENGINEERING GEOLOGY.

latter being the form wliicli the harder rocks would
assume after excavation. (See pp. 22^ 23.)

Main Brains, Bocks, etc. — In main-drainage works,
trial-holes afford ample information regarding the
kind of strata along which the sewers are to be laid,
but not as to their relation — ^a much more important
point in this particular class of work, an^ one which
is referred to further on. The remark is applicable
also to excavations for docks, foundations for dock
walls and sills, water towers, and similar works. The
evidences of the solidity of the rock, its inherent
liability to slip or to squeeze outwards under pressure,
being obtainable from trial-holes and borings, need
not here be considered.

Bridges. — In many cases the nature of the rock upon
which bridges or culverts are to be built can be very
well ascertained by trial-holes ; but not by any means
in all. For it happens that many of the largest and
most important structures, such as bridges and viaducts,
are required in the lower-lying parts of a district, that
is, in its valleys ; and here the evidence thus obtained
is apt to be misleading. The smaller valleys are, in
places, filled to a depth of many feet with a wash from
the neighbouring hills, composed perhaps of sand or
clay j which wash so closely resembles the rock whence
it has been derived, that it is, except to the experienced
eye, a part of, and continuous with, the rock of which
the high ground consists. But beneath this wash
there may be a treacherous bed of peat, or even of
quicksand, the evil influence of which is perhaps only
discovered when the new structure is sufficiently ad-
vanced for its weight to cause an awkward settlement.

GEOLOGICAL STRATA. 2T

In sucli places, it may be arged, borings are resorted
to rather than trial-boles ; even wbere sucb is the case
similar results may occur, should the misleading^
characters be repeated. And it often does happen
that in alluvial flats there is a great number of rapidly
alternating ancient river deposits, which may consist
of peat, silt, gravel, sand, or clay. The solid sub-
stratum may not be reached perhaps for fifty, sixty, or
even a hundred feet, if the spot in question be situated
over an old course of the river, sometimes a long way
from its present channel, and of which there is nothing
on the marshy plain to indicate the existence.

CHAPTER III.
GEOLOGICAL STEATA — continued.

Bearing of the nature of the rocks upon practical works, con-
tinued — Mat€7*ial8 — Minerals — Metals — Agriculture — Land-
drainage — Sewerage works.

Materials. — ^A very important element in the cost of
construction in all engineering and building works is
the material whicli the rocks of the neighbourhood
will yield, and this of course varies, both in quantity
and quality, according to the nature of the rocks them-
selves. It may be assumed that the local quarries,
lime-kilns, brick-yards, etc., will almost certainly be
on the outcrop of beds most prolific in building
materials, and afford good evidence of the kind
required. But geological knowledge is nevertheless
requisite to guide the engineer in laying out his works
so that they may strike the more valuable strata to the
best possible advantage.

Building material is frequently brought long dis-
tances, when that which is as good, or even better,
occurs in the vicinity — although perhaps hidden by a
few feet of drift — it may be in abundance. A railway-
cutting or a tunnel may be judiciously set out so as to
follow exactly the course of a useful stratum, even to

GEOLOGICAL STRATA. 29

a considerable depth from the surface, probably to
rail level. On the other hand, it may be planned so
as to miss the bed, except just at the surface, or
possibly altogether ; for the point in question depends
upon the direction of the dip of the stratum, its
consequent strike, and the actual amount of its incli-
nation.

The drift gravels occur in a more irregular manner
than any other series of deposits, but if they be pre-
viously mapped, and the work be designed accordingly,
a great saving may be effected; the labour of exca-
vating a cutting, for instance, may perhaps be made to
yield the additional result of affording ballast or road-
metalling. The extent and thickness of the gravels
should be ascertained, also their mode of occurrence,
whether as capping a ridge, filKng an old channel, or
resting on the sloping flank of a hill (see Part ii.
chap. 4). Where gravels are scarce, or altogether absent,
baUast may sometiines be obtaiiied from the more solid
rocks, broken up small for that purpose^ and in some
districts these are sufficiently plentiful. Even in many
thick deposits of clay there are found occasional beds
of hard septaria, or thin bands of limestone, suitable
for the purpose; the line of these, when their outcrop
has been traced, may frequently be followed with
advantage.

Nearly, if not quite, all the geological formations
yield some one or more forms of building material,
and many consist almost entirely of rocks that can be
utiKsed in construction. The varieties and quaUties
are numerous, still- nearly all fall under the general
terms granite, limestone, sandstone and brick-earth.

30 ENGINEERING GEOLOGY.

' Everyone who has carefully inspected a good collection of
rock specimens must have noticed tiie great numbers there are,
both of different kinds of rock and of intermediate varieties.
Distinct as they may be in some particulars, these varieties are
«till so nearly alike, that a series may frequently be selected,
presenting a perfect gradation between rocks that are distinct
if studied by themselves, and partaking more or less of the
characters of each.' {* Field Geology,' p. 152.)

There are many series of strata^ in which the rocks^
being either all limestones^ or all sandstones^ or a
mixture of both, with perhaps intervening clays, are
grouped under some comprehensive term. These
general and inclusive denominations are convenient
rather than strictly accurate; as instances may be
mentioned the Lower Oolite limestone and the Garadoc
sandstone, each series containing beds of a different
character to that implied by their group names.

In every series there are some beds of more especial
value for particular purposes than others above and
below them, although the difference may not be at
once apparent. There are beds also which for build-
ing works should be scrupulously avoided, in conse-
quence of their possessing some detrimental peculiarity
•of composition. As beds vary rapidly, that one which
is good in every respect in one district being worthless
in another, no general description can accurately apply
to all localities; therefore this point should receive
careful and local investigation.

The granites are unstratified rocks, that is, they
exhibit no lines of lamination which, in stratified rocks,
mark the original layers of deposition ; and they con-
sist mainly of crystals of quartz, felspar, and either
mica or hornblende, or they may present confused

GEOLOGICAL STKATA. 31

aggregates of those minerals in a more or less crys-
talline form. These rocks are split up by joints into
irregular tabular masses, by which the work of quarry-
ing is much facilitated.

' The durability and hardness of granite are the greater the
more quartz and hornblende predominate, and the less the
quantity of felspar and mica, which are the more weak and
perishable ingredients. Smallness and lustre in the crystals of
felspar indicate durability ; largeness and dullness, the reverse.
The best kind of granite are the strongest and most lasting of
building stones.'*

The limestones are stratified rocks of infinite variety,
generally occurring in regular series of beds, alter-
nately with clays, but sometimes as isolated beds, or
as lenticular masses, in the midst of other deposits.
They may consist of nearly pure carbonate of lime, of
a double ciarbonate of lime and magnesia, or with an
admixture of sand or clay, when they are called
arenaceous or argillaceous limestones, according to the
nature of the material which predominates. Lime-
stones are durable in proportion to their texture,
those which are compact resisting the action of
frost and the solvent power of the free carbonic acid
in the air better than those which are coarse or
porous.

Those limestones which are nearly pure are, as a
rule, the best suited to the purpose of being burnt
into quicklime, provided they are not too compact, in
which case the additional fuel required adds greatly
to the cost of calcination. An important exception is
that kind of limestone which contains from ten to

^ Eantine's ' Manual of Civil Engineering/ 1864, p. 356.

32 ENGINEERING GEOLOGY.

" '■■ ■■— ^^^ ■ III I I ■ ■ ^^^^mi^ 11 ■■■■»^»^^ I ■ ■ m.wmm ^ ■■ ■■■■-■ ■■ — ■ -^ I I fc

twenty-five per cent, of clay, and yields on burning a
cement which sets under water, and is commonly
known as hydraulic lime.

* The larger the proportion of clay in the stone, the more
rapidly the cement becomes solid, the hardeiling being com-
plete in two or three days when the proportion amounts to
twenty-five percent., and taking three weeks when only ten per
cent Much depends (especially in artificial admixtures) on
the minute division and perfect admixture of the foreign
particles.'*

The sandstones also are stratified rocks which occur
under many different conditions of texture and com-
position. They consist of quartz grains, more or less
rounded, compacted by pressure, and cemented to-
gether by siliceous, calcareous, or aluminous matter
that has been deposited within them from water
holding one or more of those substances in solution.
Upon the nature of this material, as the grains are the
same in all, the durability of the stone, of necessity,
depends. The clayey sandstones are soft and perish-
able, the calcareous disintegrate rapidly when exposed
to the weather, whilst those trith a siliceous cement
are by far the most durable. Sandstones should be
selected for building purposes which are crystalline in
texture, not dull and earthy-looking, and which do not
readily separate along their lines of lamination.

* Sandstone is in general porous, and capable of absorbing
much water ; but it is comparatively little injured by moisture,
unless when built with its layers set on edge, in which case the
expansion of water in freezing between the layers makes them
split or scale off from the face of the stone. When it is built

* Ansted's 'Geology,' 1856, p. 462.

GEOLOGICAL STRATA. '^

rate b^c^aen aro^V / /i /

on its natural bed, any water whicli may penetrate b^csreen Wei\jA /
edges of the layers has room readily to expand or escape. * >:c^ ^'^'

The better kinds of sandstone are the most generally useful
of building stones, being strong and lasting, and at the same
time easily cut, sawn, and dressed in every way, and fit alike
for every purpose of masonry.**

The brick-earths, as here considered^ include all
those clays which are, or may be, used for the manu-
facture of bricks, drain-pipes, etc. Brick-earth, of
typical quality, should consist of an intimate mixture
of pure clay with clean sand ; the proportions of these
ingredients may vary, but that of lime should not
exceed two per cent., as it causes disintegration of the
bricks ; nor is the presence of too much iron desirable,
or the bricks become vitrified in process of burning.

' Compounds of silica with one other earth are difficult of
fusion, and resist the most intense heat of a furnace . . . such
days only are fit to make fire-bricks and crucibles, and to
cement together the parts of furnaces.

' Stourbridge fire-clay consists of :

One equivalent of alumina,
Two equivalents of silica,

with two equivalents of water, and a small quantity of oxide of
iron.

Double silicates are more easily fusible ; common clays, by
the presence of silicates of lime, magnesia, and protoxide of
iron ; and the bricks made of them, when thoroughly burned,
are partially vitrified. SiHcate of , lime in the clay, in any con-
siderable quantity, makes it too fusible, so that the bricks
soften in the kilns and become distorted. Clay containing
carbonate of lime should be avoided . . . sand in moderate
quantity is beneficial ; . excess makes the bricks too brittle.
One part by volume of sand to four or five of pure cla^ is about
the best proportion.'t

* Rankine, Op. dt,^ p. 358. t Rankine, Op. cU.y pp. 364-5.

ENGINEERING GEOLOGY.

The following table gives the weight of the more
common building stones and other materials, with the
force required to crush them; those which are the
heaviest, of their own kind, being usually the strongest.
Some stones absorb much more water than others of
the same class, and are proportionately less durable ;
this peculiarity varies in granite from 1*4 to 12*5 parts
per thousand, and in hard York sandstone from 16'5
to 33* parts per thousand.

TABLE OF WEIGHT AND EESISTANCE
TO CRUSHING OF THE MORE COMMON ROCKS.

Weight per cubic
foot in lbs.

Granite -

Syenite -
Basalt -
Trap

Grauwacke
Marble -
Limestone

Crushing force
inlbe. per
square Inch.

+164 to 172* (Mount Sorrel) 12,861*

(Argyllshire) 10,917*
(Mount Sorrel) 11,820*

11,970*

Magnesian

Bath Oolite
Portland Oolite

Cha& - -

it - - -

Sandstone

„ (average)

„ (various)

Grit - - -
York flagstone

Clay - - -

Shale -

Slate - - -

Sand - - -

Gravel - - -

Shingle - - -

187*
170*

168-

156- •

169 to 175*

178*

112t
13lt
143*

- 117 to 174*

166t
144*

- 130 to 157*

131t
143+
126+
162*
tl75 to 180*
120+
]20f
90t

16,893*
6,000+

- 3,000 to 8,000+
(strong) 8,528*
(strong) 7,098*

(weak) 3,050*

3,700+
400+

5,000t

- 3,000 to 3,500*
(mean) 9,824*

* Rankine, Op» cit.

t Molesworth, Op, cU,

GEOLOGICAL STRATA. 35

Minerals and Metals. — The minerals and metals
which occur so abundantly in Great Britain, and the
cost of mining for them, are directly dependent upon
the nature of the rocks with which they are associated,
and the conditions by which those rocks have been
affected since their deposition. In the most important
instances they exist as an integral part of the forma-
tions in which they 'are found, as coal, ironstones, etc.,
but these are not, strictly speaking, either minerals or
metals. Pure minerals and native metals are com-
paratively rare, but the terms are conveniently, if
somewhat loosely, extended to include rock-masses,
or portions of rock-masses, of which certain mineral
matters, or metallic ores, form a characteristic part.
The 'minerals^ and ^ metals,^ included in the terms
thus qualified, may be for all practical purposes treated
in the same way as the rocks in which they are
enclosed, or with which they are inter-stratified.
Their existence in any given area can be ascertained
in a similar manner, their outcrop surveyed, their
extent determined, and their value approximately
estimated.

Agriculture, — ^AU agricultural pursuits are affected
to a degree much greater than is perhaps generally
understood, by the nature of the solid rocks beneath
the surface soil on which the success of farming opera-
tions is universally admitted to depend. For all soil,
or mould, has been produced, during the lapse of ages,
by the atmospheric disintegration of the surface of
those strata which form the base or subsoil. It has
been increased in depth and somewhat modified, but
its constituents have not been materially altered, by

3—2

36 ENGINEEKING GEOLOGY.

the annual growtli and decay of vegetable matter
thereon. The process has been aided by the ceaseless
action of earth-worms working into and turning up
the subsoil ; ploughing has the same effect of increas-
ing the depth of soil by constantly adding fresh
material of similar character. It is evident that the
nature of the soils of any district must therefore vary
as the subsoils or strata from which they have been
derived, and in a corresponding degree ; and that the
geology of a place being known^ the nature of its sub-
soils and soils is at the same time determined.

Gravel and sand always produce light soils, abound-
ing in silica, which may vary from fine sandy mould to
stony soil, as the particles of the rock beneath are fine
and uniform, or coarse and irregular. Clay forms
stiff, heavy, and sometimes tenacious soil, varying in
quality, but generally productive. Limestones pro-
duce light variable soils, sometimes full of detached
lumps of the rock, and generally yielding good returns
for high cultivation.

There are many physical characters which modify
the normal characters of the soils derived from sub-
aerial disintegration of the rocks ; the results of the
influence exerted by these causes may not be extensive,
but they are locally important. First may be men-
tioned the rain- wash, which results from the lighter
particles of the rocks being washed down by rain from
higher to lower ground; accumulations of this material
may often be seen several feet in thickness. Then,
where the rain-wash has been arrested by a wall or
fence on a hill-side, the result is, after a time, very
evident, in the ground being higher on the upper than

GEOLOGICAL STRATA.

•37

on the lower side. The growth of peat and the accn-
mulation of marsh-claj are agencies which give rise to
soils very different from what they would otherwise
have been at the spots beneath such influences. Again^
the soil of which a marshy plain is composed is due to
the rocks somewhat further up the valley, rather than
to the rocks actually beneath, the particles having been
brought down in suspension by the waters and de-
posited, thus forming a flat, when the stream has at
times overflowed its banks. Further, where two in-
different soils meet, which have been formed from
decomposition of contiguous rocks, that which occurs
along the line of junction is generally found to be of
better quality, owing to admixture.

Some soils are rich in fattening properties and ex-
cellent for grazing, but through want of lime, without
which no bone can be formed, young stock do not
thrive upon their produce. The deficiency can often
be supplied at a small cost, and the value of the land
be thereby much enhanced ; this is frequently the case
in low marshy situations. Other soils, having had
much of their productive properties removed by exces-
sive croppings, may be renovated by a surface dressing
of the parent subsoil ; the same object may be effected
by deeper ploughing, the subsoil being of course
gradually incorporated with the surface mould. But
for these and similar operations, certain particulars
must be obtained, or the labour so expended will per-
haps have been thrown away. These are the chemical
composition of the subsoil, and the constituents re-
quired to be added to the soil itself; details which are
readily obtainable through a ^mall expenditure of time

38 ENGINEERING GEOLOGY.

OP money upon chemical, but not necessarily quantita-
tive, analysis.

Land'd/rainage. — ^As the subsoil varies, so does the
necessity for draining the land, and the facility with
which the operation can be performed. Land-drainage,
as ordinarily understood, is a simple matter, but there
are some geological considerations respecting it to
which attention may be briefly directed. As the strata
affect the soils and sub-soils, so they must of necessity
also exert an influence upon the natural drainage, and
upon the means best adapted to that of an artificial
character. The springs of one locality being but the
natural outlet of water from another, the strata that
now throw out springs would, if occurring at a different
level, act as the channels for draining water away from
the surface to be afterwards thrown oat as springs
elsewhere. And it sometimes happens that the sub-
soils, or underlying strata, may be, by some artificial
aid, made available for purposes of drainage where
they would not so act without that assistance. In
other words, a plan of combined natural and artificial
drainage can sometimes be easily carried out where
a natural system does not exist, and where an entirely
artificial scheme can be adopted only with very con-
siderable trouble and expense.

Sewerage Worlcs. — Although the last few years have
witnessed a great, and, on the whole, beneficial change
in the methods adopted for the disposal of town-
sewage, that difficult problem is still far from being
solved. In many cases plans of irrigation have been
carried out, and these are always affected by the
nature of the rocks upon which the sewage farms are

GEOLOGICAL STRATA. 39

situated. Opinion is greatly divided^ not only as to
the respective merits of the methods of precipitation
and irrigation^ but also^ when the latter plan is in
question^ regarding the kind of soil best suited to the'
purpose.

A heavy soil will frequently yield enormous cropg
when judiciously irrigated, and in favourable seasonSi
but beyond the mechanical deposition of its suspended
particles, the water is not clarified in the process; it
runs off the land almost as chemically impure as when
pumped or discharged from the reservoir. Very little
of the liquid percolates down into a typical clay, and
the benefit derived by the crops seems to be mainly
owing to the moistening of the surface when it would
otherwise be dry and parched ; it is probable that pure
water would have almost as good an effect as liquid
sewage upon heavy land. On the other hand, light
soils absorb, and for a while retain, a great deal of the
moisture, giving it out again to the crops in a more
equable manner; they may yield less produce, but this
is, in some measure, through their containing within
themselves a smaller proportion of the elements of
fertility. It seems reasonable to suppose that as the
liquid is to a great extent filtered by its passage over
or through sandy soils, the ingredients removed from
it are ready for absorption by vegetation. A light
soil, owing to its permeability, will at all times take
more sewage liquid, acre for acre, than a heavy one,
and in some seasons — during a period of floods, for
example — ^this property may prove of great advantage.

Doubtless much may be said on behalf both of the
light and heavy soils, of the gravels and the clays, but

40 ENGINEERING GEOLOGY.

a point tliat should not be lost sight of is the ultimate
disposition of the water holding sewage particles in
solution and suspension. For considering the pheno-
menon referred to in connection with land drainage,
that the springs of one part are but the drainage of
another, it is questionable how isar we are justified in
saturating the sands of any locality with tainted water.
(See p. 38.)

There are some soils, such as loam, intermediate
between sand and clay, combining the characters of
both in proportion to the quantity of each which
occurs in their composition ; and this kind of soil may
ultimately be found the best for sewage irrigation.
Or, what is more probable, an area partly on imper-
vious clay and partly on pervious sand or gravel will
offer the greatest advantages. For in addition to be-
ing somewhat more independent of the seasons, farms
so situated would admit of the water, after running
over the clay and there depositing its suspended
matter, passing, by gravitation if possible, on to the
pervious beds. In its passage over or through these
it would be more or less filtered, and the effluent
water be thus rendered, perhaps, sufficiently pure to
be allowed to flow into a stream or river with impunity.

CHAPTER IV.

GEOLOGICAL STRATA — COfltintied.

Bearing of the relations of the rocks upon practical works —
Mining operations — Railway cuttings — Tunnels — Emhanh-
ments — Reservoirs — Canals —Main-drainage — Foundations
— Water-supply — Dampness— Disease.

The term 'relations of the rocks/ as nsed in this
work, has reference not merely to the relative posi-
tions occupied by two or more strata, although that is
the more important of their relations as generally
understood. It includes the relative numbers of the
beds or series of beds under investigation, as the
proportionate number of rocks, possessing different
characteristics in any area, exercises an important
influence in all local questions of engineering geology.
The thickness of each bed and of each series of beds,
their' constancy or variation, form also important
elements, and with these may be associated relative
surface elevation. Another is relative permeability
(although this characteristic comes under the term
the 'nature of a rock'); the relation of permeable
and impermeable beds in alternation, in juxtaposi-
tion, or witb beds of intermediate character between,
{preatly affects all practical works, when the occurrence

42 ENGINEERING GEOLOGY.

of one only, of either kind, would call for no special
investigation.

Perhaps the most important feature in the relation
of rocks to each other is the amount of dip possessed
by each, and its direction. The various and varying
dips of the rocks must necessarily bring them in con-
tact with each other under different conditions, and
must produce folds and flexures amongst themselves,
which give rise to anticlinal ridges and synclinal
troughs or valleys. When all the rocks observable in
a given area are horizontal, or are inclined at the same
angle in the same direction, they are said to be ' con-
formable,' i.e., they were deposited in regular sequence,
without any interval or break between. There are a
few exceptions to this rule, where beds not really con-
formable appear to be so, but this is owing to acci-
dental coincidence of dip, and may here be disregarded.
When one set of rocks reposes on another with a diffe-
rent dip, they are ^unconformable,^ the upper set
resting, not on the surface, but on the denuded edges
of the lower. A similar appearance to unconformity
inay be produced by an * overlap ' in strata which are
really not . unconformable, by the thinning out of one
bed, or of more than one bed, in the series.

Frequently beds are fractured, those on one side of
the crack, usually termed a ^ fault,^ having been dis-
placed vertically, by upheaval or by letting down, in
regard to those on the other side. The displacement
is called the ' throw,' and may be a few inches only, a
few feet, or even several hundred yards. Beds which
are faulted to any extent, of course are not continuous,
neither are the lower rocks where there is unoon-

GEOLOGICAL STRATA. 43

formity, and this failure in continuity exercises a great
influence, beneficial or otherwise as the case may be,
upon the extent of mineral deposits, and especially
upon the flow of underground waters.

Mining operations. — It has been shown that the
^ Minerals' and 'Metals* for which mining is carried
on depend entirely upon the nature of the rocks of
which they form a part or with which they are asso-
ciated. Upon this depends also in a great measure
the kind and the cost of preliminary borings, sometimes
incurred without the slightest prospect of success, of
the main shafts or pits, when the deposits sought for
have been discovered, and of the actual mining opera-
tions; but these are much more influenced by the
relation of the rocks in and beneath which these are
performed.

' In no respect do collieries differ more from each other than
in the quantities of water which they encounter, either in the
mining or in the subsequent working of their mineral. In one
case a retentive clay cover may prevent the access of surface
water which in another may pass in abundance through a sandy
or a gravel alluvium. In certain districts water-bearing mea-
sures of an almost fluid consistency must be passed through,
whilst in others the comparatively tight coal measures may at
once be entered. Frequently the strata above and below the
coal are so compact as to render the workings actually too
dusty and dry ; but instances are common enough in which
water makes its way through the roof stone, or through the
coal itself, and adds difficulties and expense to the whole of the
operations. When the measures through which the pit is sunk
consist of stony rock they are often allowed to stand open, but
when shales preponderate it has to be walled with brick or
stone, to which in some cases, as against the influx of water,
wood or cast iron may be preferred. But when the measures

44 ENGINEERING GEOLOGY.

are covered by other and more absorbent strata, saturated mth
water, the winning of a colliery becomes a most serious under-
taking, tasking the energies of the best men, and sometimes
collapsing after a ruinous outlay. Examples of these difficul-
ties are afforded by surface beds of sand and gravel, and by the
well-known red sand under the Ma^esian Limestone. One of
the most serious questions to be solved by the coal-viewer in
the very outset is the system by which he means to work his
mineral ; and in order to form a judgment upon this head it is
important that he should not only be acquainted with the
various modes in use elsewhere, but should have acquired a
knowledge of the peculiarities of the seams in his own district.
Where the beds have a definite dip in one direction, the work-
ing pits are usually placed as far towards the deep as it is con-
venient to go, so that underground the coal may be brought
down hill to the pit-bottom. Should the strata lie in a trough,
the pits may advantageously be placed in its middle line, so as
to command the coal on both sides .'^

Cuttings. — It is evident from a consideration of the
relation of two or more rocks to each other, as defined
above, that it may affect engineering works even to a
greater degree than the nature of the rocks on which
such works are situated. In nothing is it more
evident than in railway works that the relation of two
beds — to take a simple case — differing from each other
in kind, but having, it may be, the same dip, will be
such as to increase the cost of any work upon them,
if risk to its stability is to be avoided. Let us suppose
a railway-cutting of moderate depth to traverse two
beds of different character, one a water-bearing sand
resting evenly upon a tenacious clay, both dipping,
transversely to the line, at an angle of 3 degrees.

* Smyth's * Coal and Coal Mining,' 2nd edition, pp. 109, 111,
1^1, 122, 176 (Lockwood : 1872).

GEOLOGICAL STRATA. 45

The slope on the higher side of the cutfcing will, in a
very short time, be scored by a series of weeping
springs along the line of jonction, which will surely,
although perhaps slowly, cause serious slips unless
means be taken for their prevention. Should the dip
lie in the same direction as the fall of the ground

Fig. 1. — Railway cutting in pervious and impervious strata.
a. Pervious stratum, b. Impervious stratum.

—which is, however, unusual— the flow of water will
probably be greater at some times than at others, the
springs will, perhaps, be intermittent. Any struc-
tures, such as bridges over the cutting, must then
have their footings well down into the clay, not on it,
for its surface, even many feet from the ground-level
and under an equal thickness of solid-looking rock,
would be absolutely unsafe as a foundation.

If in the above simple instance of the effect of the
relative positions of strata (neither of which by itself
would demand special care) precautionary measures
be necessary, much more must the frequently intricate
relations of the rocks receive careful consideration.
The beds may dip much more rapidly, the water-
bearing strata may be numerous, and the geological
structure may otherwise be complicated by faults or
by local unconformity. (See also p. 42.)

Tunnels, — ^A previous knowledge of the relations of
strata to each other is still more desirable in tunnelling

46 ENGINEERING GEOLOGY.

operations ; indeed^ it would be impossible to insist
too strongly on the necessity for all geological de-
tails being known in regard to a hill to be pierced in
that manner. Not only might the style of working be
varied, but the form or strength of the tunnel itself
might perhaps be altered with advantage, to suit
either the varying pressure or the peculiarities of
rocks known to occur in the centre, different to those
exposed in the cuttings at either end. Even the
gradients might require to be modified according to
the existence or non-existence of faults or of springs
in the body of the hill, which if not previously detected
would be discovered when it is too late to make any
alteration.

If the surface of a hill be carefully examined and
the boundary lines of the beds of which it is com-
posed be • accurately surveyed, by the methods de-
scribed in Part II., their dip can then be ascertained,
and the lines of all faults laid down with a reasonable
approach to accuracy. The position of all the rocks
within the hill can then be defined as well as at the
surface, consequently the points at which they will be
met with in the tunnel can be accurately determined.
Even in cases where the all-important point of the
amount of dip cannot be obtained from actual sections,
it may be worked out — by a method hereafter ex-
plained — ^from the boundaries, or other definite lines,
if these have been laid down on the plan with preci-
sion.

In a series of beds passed through by a tunnel,
those which are uppermost generally occur near the
centre of the hill, unless they dip in one direction

GEOLOGICAL STRATA. 47

only, when the upper bed of all is, of course, at
one end. This is owing, when it occurs, not merely
to the fact of the gradient rising from each end to-
wards the interior, but to a well-known geological
phenomenon. As a general rule beds are found to
dip from each side into a hill or ridge, beneath which
they occupy a synclinal trough ; the statement being,
however, limited to hills and ridges as such, and not

Fig. 2. — Eailway Tunuel through a ridge, the beds of which
occupy a sy^nclinal hollow. The thick black line represents
a longitudmal section of the tunnel.

to include escarpments. Therefore, as the tunnel
proceeds, beds are successively pierced which are
higher and higher in the series, until the uppermost
of all that are met with is reached somewhere beneath
the axis of the elevation.

Banks. — At first sight the relation of the rocks
beneath the surface may not seem to have any direct
bearing upon railway embankments, and similar arti-
ficial accumulations of material. But there are ways
in which it does now and then greatly affect the cost
of such works, and, what is equally important, that of
bridges and culverts erected beneath them.

A line of railway does not usually run in the direc-
tion of dip of the strata, but rather at right angles to
it, consequently nearer to that of the strike of the beds,
as it follows, in a general way, the contour of the
country. The dip of the rocks beneath, whether it be

"^'' .1

48 ENGINEERING GEOLOGY.

great or small^ therefore^ is generally away from the
railway, either to the right hand or to the left, as the
case may be ; but there will be distances for which the
line of dip may more or less coincide with that of the
railway. In such cases, if the dip be of any appreci-
able amount, say exceeding five degrees, the bank,
when it attains to any height, will be very likely to
force the beds beneath it over each other along the
planes of bedding. This gives rise to slips, sometimes
of great extent, and involving much loss of material ;
they are usually sudden and liable to repetition, causing
great expenditure for pile-driving, timbering, and
other preventive measures. But if the liability to
slip, owing to the dip of the beds — ^which usually are,
in such cases, alternating clays and dissimilar deposits
— ^be previously entertained, it may be minimised by
' running the tip ahead,* and working backwards with
the bulk of the material.

If the dip be into the hill whence the material
comes, there is no risk of slips happening from this
cause, but it is occasionally the other way, when they
are sure to occur. Such slips, if quite forward, are
sometimes of slight consequence only, as the bank is
then advancing in the right direction. But should
any bridge or culvert have been built in their path,
ready for backing up, the consequences may be serious,
for such structures are then almost sure to be over-
thrown, unless the bulk of the work be done in the
backward manner mentioned above.

Besei-voirs and Canals. — The preceding remarks
apply equally to the construction of water reservoirs
and canals, but, in regard to them, it may b^ added

GEOLOGICAL STRATA. 49

that the geological structure of a country must greatly
affect the supply of water to canal-feeders and its
retention in natural reservoirs. The ' head ^ of water
that can be maintained in such reservoirs, generally
formed by a dam across some minor valley and sup-
plemented by pumping, is limited by the springs
which may occur within its area ; not by the amount
they are capable of yielding, but through other
phenomena described in Part III., which may render
useless, to a great extent, the large expenditure fre-
quently incurred for pumping. The attempt is, in
fact, often made to obtain a head of water which the
geological conditions, left to themselves, make abso-
lutely impossible; therefore, if these be understood>
and measures taken accordingly, much cost and useless
trouble may in many instances be saved.

Main Drainage, — A smaller, but not unimportant
matter, is the difficulty -sometimes experienced in
carrying out main-drainage works, owing to the sur-
face and other springs tapped by the excavations.
Several instances have occurred where, owing to the
quantity of water thus met with, the plans, after corii-
mencement of the works, have had to be altered, at
great disadvantage ; others, in which the pipes have
been imperfectly laid, or have afterwards settled in the
quicksands, so that the joints have ever after been
unsound. In consequence, they have admitted ' the
spring waters, and thus added, perhaps, several
hundred pounds a year to the cost of pumping, besides
deteriorating the value of the sewage for irrigation or
precipitation. These results have happened, in some
cases beyond hope of remedy, not because the geologi-

50 ENGINEERING GEOLOGY.

cal details, including a knowledge of the springs, could
not have been ascertained in time, but simply because
they have been ignored. The remedy for such a state
of things, where one is practicable, is to be sought in
the plan described in Part III., but it must always be
adopted within certain limitations, and, indeed, should
only be allowed under official supervision.

Foundations. — One observation may here be made
with regard to foundations, whether of dock works,
bridges, or buildings. From the preceding notes it
will have been seen that however solid the stratum
may apparently be in which the excavations are made
for foundations; the calculations as to stability are
incomplete and liable to error — as the works are to
unforeseen catastrophe — ^unless all the relations as well
as the nature of the rocks beneath have been ascertained
and taken into consideration.

Water Supply. — The question of supplying the in-
habitants of towns and villages with good drinking
water forms one of the chief problems of our day. It
is essentially a question for engineers, but a knowledge
of geology is indispensable to him who would attempt
its satisfactory solution. An immediate improvement
in the water supply of any district, and in its sanitary
conditions generally, is one of the practical results
that may reasonably be expected to arise from the
working out of its geological structure — that is, from
a knowledge of its rocks and of their relation to Bach
other. For the available supply of water in any given
locality is not by any means proportionate to its rain-
fall ; as by the widely spread water-bearing beds it is
to a great extent equalised. The supply to be obtained

GEOLOGICAL STRATA. 51

by boring down to deep-seated springs is practicjally
inexhaustible, being scarcely, if at all, affected by
drought, and these springs form the only source on
which can be placed a full reliance.

The phenomena of springs, and of the sources of
supply to Artesian wells, both of which are practically
important, are entirely dependent on stratigraphical
and physical features. An explanation of them has
been given in Part III., as well as of the reason why
sometimes salt waters occur far inland, and fresh-
water springs beneath the sea ; why some waters are
chemically pure, whilst others are saturated with
mineral salts. And it may be added that the localities
are few beneath which such springs may not be found
at a greater or lesser depth, according to their physical
^nd geological features.

Dampness, — ^Another point affected by exactly similar
conditions to those that govern water-supply is the
dampness of a locality ; a condition which sometimes
renders almost uninhabitable what would otherwise be
a desirable and healthy situation. It may arise either
from the physical conditions of elevation, situation,
rainfall, and so on, or from those of a purely geo-
logical nature. Frequently it is owing to the saturated
condition of the water-bearing beds immediately be-
neath, and probably close to, the surface; in other
words, by the proximity to the ground level of the
general water-line of the district. This is a question
that should influence the choice of situation for all
public buildings, and, indeed, for private houses also,
where the circumstances are such as to admit of selec-
tion. It is fully considered in Part III., a section

4—2

52 ENGINEERING GEOLOGY.

being devoted to the subject of ' Sites,' as one which
merits, but seldom receives, much attention.

Disease. — ^A knowledge of the undoubted relation
existing between subsoil and disease must also be bene-
ficial, especially to those who may be seeking for them-
selves a new home. This question cannot be fully
entered on here, but the results of certain official
inquiries into one branch of the subject are appended.
In the 'Report of the Medical Officer of the Privy
Council,' for 1867, pp. 14—17, and 57—110, is dis-
cussed, from several points of view, the interesting
question of the connection between the geological
structure and the consumption death-rate of a district.
After careful consideration of all the facts and statis-
tics adduced, the following suggestive and valuable
conclusions were arrived at, and may be considered as
fairly well established :

(a.) That on pervious soils there is less consumption
than on impervious soils.

{!),) That on high-lying pervious soils there is less
consumption than on low-lying pervious soils.

(c.) That on sloping impervious soils there is less
consumption than on flat impervious soils.

{d.) These inferences must be put along with the
other fact, that artificial removal of subsoil
water, alone, of various sanitary works, has
largely decreased consumption. From which
follows the general inference, that wetness of
soil is a great cause of consumption.

If this one disease can be so influenced that its
ravages in a district may be, as they have been.

GEOLOGICAL STRATA. 53

lessened one half by a simple draining of the land, it
may reasonably be assumed, that the power of other
diseases also is more or less dependent on certain
physical conditions, which are susceptible of natural or
artificial modification.

riNMYi:Ksrrv of 1
CALlFOiiNlA..

PART II.

PROCEDUEE IN THE FIELD.

CHAPTER I.

METHODS EMPLOYED IN GEOLOGICAL SURVEYING. MAPS

AND SECTIONS.

Methods employed in geological surveying — Determination of

rocks — Tables.

Methods employed in Geological Surveying, — Geology,
as a science, relates the history not only of the rocks
of which the earth^s crust is composed, that follow
each other in regular if somewhat disturbed sequence,
and form the indestructible pages of a record on which
that history is imprinted. It describes also the succes-
sive races of animals and plants that have flourished
on its surface or in the ocean, and whose remains, now
petrified, were enclosed in the rocks at the time of
their formation. An acquaintance with the organic
remains enclosed, as fossils in the strata, is therefore
of great scientific value in determining the geological
age or position of the rocks, but it is to a great extent
unnecessary for practical works. What is required
for mechanical operations and for sanitary purposes is
a knowledge of the stratigraphical geology of a dis-

PROCEDURE IN THE FIELD. 55

trict ; that is to say, of the nature of the local rocks
and of their relation to each other.

If we would make a series of drawings to show the
geological structure of any locality, we must first
trace upon a map the boundaries of its rocks, and thus
define the area that each formation occupies. We
must next ascertain the angle at which the rocks dip
away from the surface, and then, aided by our notes,
construct a section, which shall portray their under-
ground extension and relative position. We must
further ascertain the kinds of rock of which the beds
consist, by their general appearance and the aid of
simple tests in the field ; or if necessary, by others,
more complicated, applied to detached specimens at
home. Thus, to form an accurate idea of the rocky
structure of a district, and to represent and describe
its geological features, three operations have to be
performed : —

(1). The character and peculiarities of the strata
which crop out at the surface determined.

(2). The boundaries of the different rocks laid down
upon a map.

(3). The dip, if any, and the underground continu-
ation of the beds worked out and shown upon
a section.

The methods of geological surveying adopted in the
carrying out of these operations are described in the
following order : — Detemnination of rocks ; Geological
maps ; Geological sections.

Detemiinatimi of Rochs, — It is obvious that for
engineering purposes a great deal depends upon the

o6 ENGINEERING GEOLOGY.

physical characters of a rock, whether regarded as a
building material in itself, as a scarce whence such
material is derived by any process of manufacture, or
as a substance possessing any influence upon con-
struction, stability of work, or any other practical
considerations. Therefore it is highly necessary for
the geological engineer to be able not only to deter-
mine the class to which any particular specimen belongs,
but also to form some reliable ideas as to what sub-
stances enter into its composition. There are simple
tests, for application in the field, to ascertain the class
of rock under examination ; and more delicate tests,
by which the field results may be checked and extended.
In most cases the directions given below will go far
towards accurate determination ; but it will be advisable
to consult works devoted to the subject, in the exami-
nation of difficult specimens.

To ascertain the kind of rock exposed in a pit or
quarry, a fragment is detached from that part which
has been least altered by the action of the weather.
A good-sized piece of the rock is broken ofE, and
afterwards reduced by chipping into a square lump;
good edges are thus obtained for observation of its
texture by the aid of a pocket-lens or magnifier.

In the field, the first test may be made for hardness
— a character determined with reasonable accuracy by
the facility or difficulty with which the specimen can
be scratched, if at all, by a pocket-knife. The result
will show to which of the. three main divisions of the
following table the rock may be referred — scratched
with ease, with difficulty, or not at all. It may then
be tested for effervescence by dilute hydrochloric acid.

PROCEDURE IN THE FIELD. 57

a small bottle of which is always carried for that pur-
pose in geological surveying. The fact of its efferves-
cing rapidly^ slowly, or not at all, will narrow still more
the limits of the list in which it is included. Further
determination is then made by the additional tests of
texture, colour, etc.,, given in the second column of the
table, which in most cases will settle at least the class
to which a mineral or a rock belongs. The texture is
observed at the chipped angle of the specimen ; it may
be crystalline, glassy, compact, earthy, granular, or
laminated. The peculiarities of fracture, lustre, and
streak are valuable aids in a precise examination, but
are omitted from the table for the sake of greater
simplicity, as without them results may be obtained
sufficiently near for all practical purposes.

The behaviour before the blow-pipe of many sub-
stances is given, to distinguish some which cannot
"otherwise be separated. This instrument is of great
assistance in discovering the exact character of rocks
and ores ; a microscope also is invaluable for the same
purpose. There are some rock substances which may
be chemically analysed without much trouble; but
works, like those named below, specially devoted to
blow-pipe, microscopic, and chemical analysis should
be consulted for the methods of determination of the
ultimate constituents of a specimen.

The Sttidy of Roch. Rutley (Longmans).

Introduction to the icse of the MotUh Blowpipe, Scheerer and

Blanford (Williams and Norgate).
Chemical Geology. Bischoff.

58 ENGINEEKING GEOLOGY.

TABLES FOR APPROXIMATE DETERMINATION OF
MINERALS, ORES, AND ROCKS.

Field Tests. Additional Tests.

1. Those which are easily Texture, usual colour, heha-
scratched- by a knife, viour before the blow-pipe, etc.

and —

(a) Effervesce rapidly,
are—

Calcite (carbonate of lime) Crystalline, white or tinted^

infusible, reduces to quick-
lime
Satin spar (carbonate of lime) Fibrous „ „

Marble (carbonate of lime, with Crystalline, various colours

some silica, alumina, etc.)
Limestone „ „ Compact, sometimes oolitic „

Chalk ,, „ Earthy, white or pale yellow

Galena, lead ore (sulphide of Crystalline, dark grey colour,,
lead) decrepitates and fuses,

yielding metallic globule.
Chalybite, iron ore (carbonate Crystalline, brownish colour^
of iron) blackens and becomes mag-

netic
Clay ironstone „ (impure „; Concretionary, brown „ „

(6) Ejfh'vesce slowly.

Dolomite (carbonate of lime and Crystalline, white or tinted,.

magnesia) infusible, reduces to quick-

lime
Magnesian limestone („ „ Compact or granular, greyish

with some silica, alumina, etc.) colour

Selenite (sulphate of lime) Crystalline, white, exfoliates,

becomes opaque

Oypsum „ „ Compact or minutely crystal-

line, white or tinted „ „

PROCEDURE IN THE FIELD. 59^

Fluor spar (fluoride of calcium) Crystalline, white or purple-
tinted, fuses to a clear-
bead, which on cooling be-
comes opaque

Sandstone, micaceous Granular or laminated (lami-

nae glistening), various
colours

Hornblende schist Foliated, black

Glauconite (silicate of iron and Compact, olive green, fuses
potash) to magnetic glass

Chlorite (silicate of magnesia, Compact, foliated or granular,,
alumina, and iron) dark olive green, fuses on

thin edges with difficulty

Chlorite schist (chlorite, quartz. Foliated, green
etc.)

Mica (silicate of alumina, etc.) Various colours, plates elastic,.

fuses on thin edges only

Mica scliist (mica, quartz, etc.) White or green (folia hard and

glistening), dark grey

Talc (bisilicate of magnesia) White or green, plates not

elastic, fuses on thin edges
with difficultv

Steatite, * soapstone ' Compact, whitish, fuses with

difficulty

Serpentine rock (silicate of Compact, dark olive-green,
magnesia) fuses on thin edges

Fuller's earth (silicate of Earthy, greenish-brown, fuses
alumina) to porous slag

Slate „ „ Compact (cleaved), dark grey

Barytes (sulphate of baryta) Crystalline, white or tinted,.

decrepitates and fuses with
difficulty

Blende, zinc ore (sulphide of Crystalline, black or brown,,
zinc) infusible alone

Blue vitriol (sulphate of copper) Crystalline, with soda yields

copper bead

Copper glance, copper ore (sul- Crystalline, lead grey, fuses^
phide of copper) yielding copper bead

<50

ENGINEERING GEOLOGY.

Copper pyrites, copper ore (sul-
phide of copper and iron)
iJopperas (sulphate of iron)

Bock salt

Graphite, black lead (carbon)

Coal

(impure „ )

Compact, brass-yellow, fuses,
yielding magnetic bead

Compact, green, fuses witli
borax yielding a green glass

Crystalline, tinted, decrepi-
tates ancL fuses

Crystalline, foliated, black,
infusible

Compact, black

2. Those which are tvith diffi-
culty scratched by a knife,

-and —

(a) Effervesce rapidly,
•Calamine, zinc ore (carbonate of Compact, greyish, infusible
zinc) alone, fuses easily in borax.

(6) Effervesce sloivly.
Limestone, siliceous

Felspar, oligoclase (silicate of
alumina and soda)

Felspar, common
Hornblende (silicate of lime,
magnesia, etc.)

Hornblende rock

Augite (silicate of lime, mag-
nesia, etc.)

Hypersthene (silicate of mag-
nesia, and iron)]

Compact, various colours

Crystalline, sometimes com-
pact, white or pink ; fuses
on thin edges only
„ „ white or tinted,
fuses with difficulty to clear
glass

Compact „ „

Crystalline, green, brown, or
black, fuses easily to mag-
netic globule

Compact, green, brown, or
black, fuses easily

Crystalline, greenish-black,
fuses easily to grey glass

Crystalline, brownish-green,
fuses easily to black enamel

PKOCEDUKE IN THE FIELD. 61

Hypersthene rock, 'greenstone' Crystalline, greenish-black,.

(oligoclase and hypersthene) fuses easily

Felsite (orthoclase^and quartz) Compact, grey, weathers white

Dolerite (oligoclase and augite) Crystalline,granular,dark grey

Basalt Compact, black

Diorite, * greenstone' (oligoclade „ green, weathers

and hornblende) brown

Porphyrite (oligoclase and horn- „ pink, brown

blende, etc.) '

Qabbro, 'greenstone/ (oligoclase Crystalline, greenish

and diallage

Trachyte Compact (feels rough), grey

Obsidian Glassy, brown or grey

Phonolite, * clinkstone ' Compact, grey, weathers white

Apatite (phosphate of lime) „ infusible

* Coprolite ' „ Concretionary, brown

Specular iron, iron ore (per- Crystalline, steel grey, infu-

oxide of iron) sible alone

Haematite „ „ „ Reniform,red

Limonite „ ' brown hae- Compact, brown
. matite '

3. Those which cannot he
scratched by a knife,

and —

Do not effervesce.

Granite (quartz, felspar, and Crystalline

mica)
Gneiss ( „ „ „ in folia) „

Syenite ( „ felspar and horn- „ more durable than

blende) granite

Quartz (silica) „ white or tinted, infus-

ible alone
Quartzite (fine grains of quartz Compact, granular

in siliceous matrix)
Quartz sandstone („ „ if „ , ,

a slow effervescence the

matrix is calcareous)

*62 ENGINEERING GEOLOGY.

Flint (silica, not quite pure) Compact, black or grey
Hornstone „ „

Chert „ various colours

Iron pyrites (bisulphide of iron) Crystalline, bronze-yellow,

fuses to globule attracted

by magnet

CHAPTER II.

METHODS EMPLOYED IN GEOLOGICAL SURVEYING

{ccyiitiniced) ,

Construction of geological mB.j^8— Geological surveying —

Example — Dip.

Geological Maps. — It is very essential when tracing
and mapping geological boundaries to have a good
map of the district to be thus surveyed, for, however
accurately those lines may be laid down, any errors
on the map must a£Eect the after calculations. All the
physical features of the district should be distinctly
indicated on the map, such as hills, rivers, and streams,
and heights above the sea level in figures here and
there are of great advantage. Maps drawn to a scale
of 1 inch to a mile answer well for general purposes ;
if great accuracy be required, those on a 6-inch scale
should be used. Both kinds are issued by the Ord-
nance Survey ; of the whole country on the smaller
scale, but on the larger scale those of a part only have
up to the present time been published.

Contour lines are engraved on the 6-inch maps, and
running, as they do, through all the points where a
horizontal plane at any given height would intersect
the surface of the ground, are of great assistance in
geological surveying. To the eye accustomed to them
these lines convey at a glance the physical geography.

64 ENGINEERING GEOLOGY.

or the actual shape of a tract of country, its hills and
valleys, its precipices and ravines. Contours, of course,
run in a V-like shape up the valleys, in straight lines
on even flanks and ridges, and sweep in curves round
the outlines of the hills ; their variations are numerous
as those of the features themselves, but these kinds of
form prevail in all.

The three following propositions afEord considerable
aid to those engaged in geological surveying :

(a.) The boundary-lines of horizontal strata exactly

coincide with the contours.
(6.) The boundar^r-Unes of strata dipping towards a

hill are less winding than the contours,
(c.) The boundary-lines of strata dipping from a

hill are more winding than the contours.

Therefore :

(a.) One point through which the boundary passes
being ascertained, the line will thence follow
the contour exactly, so long as the beds con-
tinue horizontal.

(fe.) In this case every point on the line of strike, at
the same level as one through which the
boundary passes, must be also on the line of
boundary. A line somewhat less curved than
the contour, being flattened in proportion ta
the dip, represents the line required.

(c.) When the strata dip with the slope, the line must
be discovered at several, points, and these
united by exaggeration of the contour, but
with the windings reversed when the dip
exceeds the slope of the ground.

PROCEDURE IN THE FIELD. 65

Oeological Surveying. — Useful as the above propo-
sitions are, the ground must nevertheless be gone
over, and the actual line followed, for dip may change
where alteration is least expected. Faults also may
occur ; these will suddenly terminate a boundary and
introduce a fresh line, representing the broken ends of
the strata that have been upheaved on one side of it,
or thrown down on the other,

A geological map is one which, as we have seen,
defines the area occupied by the surface of each forma-
tion, or by its denuded edge where it comes to the
level of the ground. It follows that to construct such
a map with accuracy every part of the surface within
any area to be geologically surveyed must be exam-
ined. If by any means the nature of the rocks over a
given area be proved, say by borings or trial-holes,
one in every acre of ground, and the varying results
be shown by different colours, a geological map would
be roughly presented. But it would be an approxima-
tion only, for the lines of division between the rocks
would still remain to be shown.

Similar evidence to that afforded by trial-holes may
be obtained, in many other ways and with much less
trouble, as to what is the uppermost stratum at any
given point, or any number of points ; and for our
present purpose it is in regard to this stratum only
that the information is required. Boad-side banks
and ditches, even of moderate depth, will all yield evi-
dence as to the kind of rock at the particular spot
where examined. It is obtained by picking into the
bank, or the side of the ditch, and by cutting at the

66 £NGIN££BING GEOLOGY.

£ace of each exposed section; but in all cases
the actaal stratum beneath the soil and rain-wash
must be determined. In the absence of all such
sections, the surface-soil may be turned aside^ here
and there, along the probable line of boundary. The
heaps of diflferent stuff thrown out from iheir holes
by moles and rabbits will afford indications, and
search in ploughed fields will generally lead to the
discovery of lumps of the rock being traced. Lastly,
the eye, by practice, will enable us to judge from the
soil itself what is the rock beneath, from which it has
been formed by a process of disintegration. Another
important point to be noted is the existence of springs,
as these indicate not only a change from pervious to
impervious strata, but also the actual lines of division.
All the results of the examination of the ground must
be entered on the map by some mark or symbol, in
the exact situation of each observation, and from these
the geological lines will be drawn.

Where a rock comes to the surface, the area it occu-
pies is bounded by two lines : one is called its ' line of
outcrop,' and coincides with its upper edge; the other
coincides with its lower edge, and is called its ' line of
boundary.' The former is of course the line along
which it crops out from beneath a higher stratum, the
latter marks its lower and outer margin or boundary,
and is the same as the line of outcrop of the rock im-
mediately beneath. Sometimes the geological will coin-
cide with the topographical lines, but it is not often the
case, as the intricate windings of the natural divisions
of the rocks adhere, more or less closely, as previously
explained, to the contour lines of the country.

PROCEDURE IN THE FIELD. 67

Traversing. — In walking over a district for the pur-
pose of mapping its strata^ it is well to follow some
regular plan. The lines of ditch and fence may be
taken up and down alternately ; if too far apart, they
can be left, and the ground between walked over in
search of evidence. By thus following one fence, for,
say, hair a mile in any direction, and returning by
another the width of one field apart from the first, and
by repeating the process at the distance of another
field, all details concerning a considerable area may be
collected. This plan may involve the drawing of
several geological lines as the boundaries of the strata
that occur are crossed and recrossed. It is frequently
found more convenient to commence and follow out
one line only at a time ; this is done by walking along
a ditch or fence until the line is discovered, crossing
the field, and following the next fence until the line is
again crossed, and so on to its termination.

Example of Oeological Surveying, — The nature of
some of the rocks occurring in the district to be sur-
veyed having been previously determined, their relation
will be worked out, and the area occupied by each
defined by drawing its boundary-line in the following
manner : — Let it be assumed, for sake of illustration,
that the evidence obtained, whether by trial-holes or
by any of the preceding methods, has established the
existence of limestones over all the higher ground, of
clay upon the flanks of the hills, of ferruginous sand-
stone in places along their foot, with clay again in the
plain beyond.

Starting from some point on the table-land, and
following a line of fence, the surveyor notices that the

5—2

68 ENGINEERING GEOLOGY..

soil is Ughfc, and full of fragments of a whitish lime-
stone, varying in size from small grains to lumps as
large as a hen^s egg ; and in the ditch he occasionally
sees the beds of limestone in their undisturbed position.
Just below the point where the ground begins to
decline towards the valley he observes, in a slipped
portion of the bank, rock of a more sandy nature, full
of concretions of a yellow, rusty-looking substance,
which turns out to be a workable ironstone. Near
this point is a small spring, the water from which has
worn away the ground into a hollow and deep channel,
ejcposing the blue clay just below. It is evident that
the pervious limestones overlie the impervious clay,
by which the water that has percolated through them
has been thrown out on the hill-side ; also that there
is between them, a bed of ironstone of slight thickness.
The line of division, of course, passes through this
spring, the exact position of which must be fixed on
the map, by compass-bearing or otherwise. A short
line is then drawn in pencil from this point to either
hand, in the same direction and of the same form as a
contour-line would have at that elevation.

This line of boundary will again be met with in
returning by the next fence, although it may not again
be exposed by a slip or indicated by a spring; but
when the surveyor arrives near the spot through which
he expects it to run he seeks for it by some of the
methods already described. He may not be able
actually to see the junction of the beds, but he will
presently be at a spot where limestones occur just
above him and blue clay below, and* here, of course,
must be the boundary. At this spot is, perhaps, a

v«

PROCEDUEE IN THE FIELD. 69

small plantation^ a bend in the fence^ or some other
mark shown on the inap. He will draw contour-lines
from this point also to either side, one of which
lines will be found to unite with that drawn from the
first fence in this direction. It is for the present
assumed that the beds are horizontal : should it prove
otherwise, the pencil lines, if curved, must be flattened
or exaggerated, accordingly as the dip is into or
away from the hill.

But to return to the line of fence along which the
mapping was supposed to be commenced. After
passing the spring, the surveyor walks for some dis-
tance over a clay of bluish colour, which is seen every
few yards in the ditch that runs straight down the
hill. The soil is hard, and in many places cracked;
this would sufficiently indicate the nature of the stratum
beneath, even were it not visible as it is in the sides of
the difcch. Towards the lower part of the slope the
soil gets lighter, being covered by a sandy wash from
the hill above, and presently a pond is met with, which
shows, when its margin is picked into, signs of another
bed of sandy ironstone. This is noted on the map by a
symbol in its exact position, and left for the present;
it may be of trifling local occurrence, or it may be
part of an important deposit. Beyond this is blue
clay again, covered by a foot or two of sandy wash,
but exposed wherever a ditch, pond, or other exca-
vation through the surface-soil lays it open to
inspection.

Beturning by the secojid fence in the direction of
the high ground, an exactly similar, but reversed^
sequence of strata is observed. There is, however, a

70 ENGINEERING GEOLOGY.

brick-pit situated just at the foot of the slope, in the
vertical face of which is seen a bed of calcareous and
ferruginous rock, two feet thick, with blue clay above
and below. This bed is evidently continuous, judging by
contour, with that noticed in the pond, therefore a line
is drawn to connect the two points, and following the
shape of the ground. Continuing his walk up the
hill, the surveyor again traverses the blue clay until he
reaches the small plantation above mentioned ; beyond
this point he meets with limestones only, and in these
there ard iseveral quarries on the table-land.

The sequence of beds in the area covered by this
short walk is thus established, as : —

Limestones, in horizontal beds.
Ironstone, thickness not proved.
Blue clay.

Bed of Ironstone, 2 feet.
Blue clay.

And in this order of succession they would b6 met
with in a boring commenced on the higher ground.
The thickness of each would be determined without a
boring, if a section of the surface were taken, to show
the position of their outcrop, for the beds are, so far
as we have seen, horizontal, and present no signs of
unconformity.

In this way simple boundary-lines are discovered
and drawn ; they follow the form of the ground, which,
indeed, is mainly due to the varying hardness and
changing dip of the rocks where they rise to the sur-
fece. For, in accordance with those characters, the de-
nuding agencies have worn away the rocks ; that is to

PBOCEDUKE IN THE FIELD. 71

say, the minor features of a district are hills or hollows,
according to the power its rocks possess of resisting
denudation. As dissimilar rocks must thus make
a change of feature along their junction, a knowledge
of the fact is, as we have seen, turned to account,
and is of great service, in drawing their lines of
boundary.

CHAPTER III.

METHODS EMPLOYED IN GEOLOGICAL SURVEYING —

continued.

Dip and Strike — Eules for finding dip — Example.

Bvp and Strike. — The term ^Dip' has frequently
been used, and it is scarcely necessary to explain that
it means the angle at which the bedding planes of
strata are inclined to the horizon. A vertical plane
intersecting the bed where it makes the greatest
angle with the horizon, is in the * direction of the dip/
Another line, at right angles to this, must necessarily
coincide with that part of the rock which is horizontal,
and would be seen in such a position in the face of a
quarry cut back in the dip^s direction. This second
line is called the ^ strike ' of the rock, and if the sur-
face of the ground were a level plane the outcrop of a
stratum would always coincide with the strike, and be,
throughout its entire length, at right angles to the
dip. But as the surface is uneven, the outcrop winds
accordingly, crossing and recrossing the general line
of strike, which must perforce remain the same until
the dip assumes an altered direction. And as the dip
varies, so does the width of outcrop of beds or forma-
tions of a given thickness.

Clinometers are generally used for measuring the

PBOCEDURE IN THE FIELD. 73

angle of dip, but other instruments answer the purpose
equally well if they indicate the amount of inclination.
It is most frequently in mines, quarries, brickyards and
similar places, that sections of the rocks are seen, but
it should be remembered that these exposures may
not be in the true dip^s direction, and that the apparent
dip only can thus be taken. This must be observed
in two or more faces which make a considerable angle
with each other, the lines of their direction being laid
down on paper ; the result may then be worked out by
the rule given below.*

Note. — The true dip may be greater, but it

cannot be less, than that seen in any face of a

section open to observation.
Rule, — ^When two observed dips incline from or
towards the angle enclosed by their lines — A B, A 0,
in fig. 3, p. 75 — the true dip is at right angles to a
line thus laid down : — Set off from the angle on each
one of the two lines of apparent dip, a number of units
corresponding to the number of degrees of dip observed
along the other line. A line connecting the two points
coincides with the strike, and is consequently at right
angles to the true dip^s direction. Instead of the
angle of dip, the proportionate incline may be set off
on its own line, the result obtained, of course, being
also a proportionate incline.

When one observed dip inclines from and the other
towards the angle — as A D, A C^ fig. 3 — the true dip
can be worked out by prolonging one of the lines of
apparent dip beyond the point of convergencie; both

* Published by the author in the Geological Magazine^ May,

187a

74 ENGINEERING GEOLOGY.

■

apparent dips will then run either from or towards the
angle thus formed, and the true dip can be found by
the above rule.

Should the amount of dip be considerable, and great
accuracy be required, set ofE the tangent of the angle of
apparent dip. For, except in the smaller angles, there
is an error arising from the difference between their
circular measure and their tangent, but in most cases
this is too slight to be of any practical value.

When the direction of a dip has been thus worked
out, ifc becomes necessary to ascertain its amount, also,
from the apparent dips observed in the quarry. From
any two observed dips the amount and direction of
the true dip may be obtained by calculation, but for
all practical purposes the results arrived at in the
following manner are sufficiently accurate and much
more expeditious : —

Construct on one of the observed lines a right-angled
triangle representing the apparent dip, its base being
equal to the length set off along that line. The per-
pendicular is equal to the amount which the bed falls
in a distance equal to the length of the base of the
triangle. The bed falls the same amount in the
shorter length at right angles from the line of strike
(previously drawn) to the angle of the quarry.
Another triangle constructed wifch this shorter base
and the same perpendicular gives the exact measure of
the dip required.

The true dip and its direction may also be worked
out, and the existence of two different dips discovered in
any locality, where no sections of the beds are exposed.
This is done by first accurately surveying some definite

PROCEDURE IN THE FIELD.

fiO P

Fig. 3.— Diagram to illustrate the Rule for working out the true
dip, from observed apparent dips — as angles or propor-
tionate inclines — and from three points on an outcrop.

A By 75 feet in 200 yards— Incline 1 in 8.

AC, 63 „ „ 400 „ = „ linl9.

76 ENGINEERING GEOLOGY.

line^ such as a thin band of rock^ or a line of division
between two beds, and by then taking the relative
heights of three of the most suitable spots upon
it for that purpose. These spots should be so
chosen that lines connecting them will form a
near approach to a right angle; the distances
apart must be either measured or scaled from the
map; the heights can be taken by a level, or by an
aneroid if extreme accuracy be not required. The
points thus selected will afEord the same information
as if the stratum along the lines connecting the points
of observation were exposed in two faces of a quarry.
The lines represent the direction, and the difEerences
in height will give, with the lengths, the proportionate
incline or, by aid of a diagram, the amounts of the two
apparent dips, from which the amount and direction of
the true dip can be found by either of the methods
described.

Example. — The following is an example of working
out dip from three points of outcrop by means of
proportionate incline : —

Three points, A, B, 0, are on the line of outcrop of
a definite stratum ; A is 200 yards (measured W.
3° N.) from, and 75 feet above, B; 400 yards
(measured S. 5° B.) from, and 63 feet above, C.

The fall from A to B is thus found

to be at the rate of . . . 1 in 8
The fall from A to C at the rate of 1 in 19

Lay down these lines on paper, as in fig. 3, and
Set ofE on A, B 8 units of length.

^> » A, O 19 „ y,

PKOCEDURE IN THE FIELD. 77

Connect the twopoints thus set off, by a line that
will be in the direction of the strike, and at right
angles to that of the true dip, which proves to be
W. 26° S.

The amount is thus ascertained : —

The bed fe,lls at the rate of 1 in 8 along A, B —
1 foot (or other unit) in the 8 set off along that line
(and similarly 1 in the 19 along A, 0) — it falls an
equal amount in the shorter length of a perpendicular
let fall from the angle at A to the line of strike already
drawn : that length scales 7 units, giving a proportion-
ate incline of 1 in 7 (equal to 8°), in the case in question.

As dip may vary rapidly, this is not always a strictly
accurate method, but with care it will give approxi-
mate results in all cases, and is especially valuable in
the surveying of districts for the estimation of their
water-supply.

For a description of the instruments used in geo-
logical surveying and for tables of dip, proportionate
inclines, etc., the reader is referred to the treatise
previously mentioned — 'Field Geology^ — which is
intended more especially for geological students, and
in which the subject is treated at greater length than
is considered necessary in a work designed for those
to whom such instruments are necessarily familiar, /^^^i-

CHAPTER IV.

METHODS EMPLOYED IN GEOLOGICAL SURVEYING —

continued.
Example of surveying faulted area — Drift deposits.

•

Example of Geological Sui'veying, — A somewhat more
complicated example of geological surveying may now
be given, involving the application of the. methods
described of determining the nature of rocks, of ob-
serving dips, and of tracing irregular boundaries,
faults, and unconformities. The surveyor usually
obtains his evidence for the lines and draws them at
the saipe time ; in addition to this, he also, in the same
traverse, notes all exposed sections of the rocks,
measures all dips, determines the nature of the various
beds, and coUects^all other geological information.

The area to be surveyed is a mile wide from west to
east, and a mile and a half in length from south to
north ; its main physical features are a ridge of high
ground on the south, sloping generally down towards
the north, with a stream running in an easterly direc
tion across a depression in the centre. It is shown as
a completed geological map in fig. 4, p. 80, from which,
however, the roads and other topographical details
have been omitted, in order that the geological lines

PROCEDUEE IN THE FIELD. 79

may be more easily followed in the description of the
surveying operations.

Commencing near the north-east corner and walking
in a westerly direction, it soon becomes plain to the
surveyor, from the nature of the soil, that a sand, or
sandstone, forms the stratum beneath. There are no
ditches, a fact which also indicates subsoil of a light
description, and one that requires no draining; the
absence of these, however, prevents, for a time, any
other than surface indications being obtained. But a
quarry, No. 1, is presently met with, and its exact
position is laid down on the map as ascertained by
compass-bearing. One observation is taken upon
some distant object, perhaps a church ; another upon
a second object, the lines crossing each other at an
angle, as nearly as may be, of 90**.

The face of the quarry No. 1 shows the following
beds in descending order, which are thus entered in
the note-book : —

Brown and reddish-coloured freestones
in beds 1 ft. to 2 ft. thick, with part-
ings of sandy clay . . . .10 feet

Red marl, with lumps of a white and pink

crystalline substance . . . . 2 „

Red freestone, as above . . . . 4 „

Red marl, not bottomed . . . . 3 „

The apparent dips are to N.W. 3' 20', and N.N.E.
4° 20'.

The dip, worked out, by the rule given in p. 73,
from the apparent dips observed along two sides of
the quarry, proves to be due north 5**, this is entered

ENGINEEBING GEOLOGT.

Rg 4 — Map of Area geologically snrvered

PKOCEDURE IN THE FIELD. 81

Ebfeeence to Pig-. 4.
Glacial Drift. E3 Boulder Clay.

Middle Permian. ^sj Magnesian limestone.

Lower Permian. ^M Sandstone.

Upper Carboniferous.

>>vsi Coal Measures,
^f^ Limestone.
Coal-seams.

Txl Millstone Grit (not seen in
map).

Felsite dyke.
Recent. p;;^] Alluvium.

on the map in its right position, the amount in figures,
with an arrow pointing in the dip's direction. This
direction is transverse to that of the general shape of
the ground, consequently the feature, or contour-line
— modified as explained on p. 64 — will be found to
correspond, or nearly so, with the strike of the beds,
or possibly with, the boundary of the formation to
which they belong.

The freestones are easily scratched by a knife, and
do not effervesce upon application of the dilute acid;
they have a very fine granular texture, with glisten-
ing particles in rather indistinct lines of lamination.
Prom these characters the beds are concluded to be
micaceous sandstone; the partings of shale are cal-
careous, being slightly effervescent. The pinkish sub-
stance included in the red marl is equally soft, and
does not effervesce ; it is minutely crystalline, as seen
Tinder the lens, and proves to be gypsum. (Table, p. 58),

82 ENGINEERING GEOLOGY.

Continuing to advance in a westerly direction from
the quarry, the surveyor finds that near the edge of
the map the sandy soil disappears^ and is replaced by
one which is darker in colour and more clay-like in
character. A little to the south many lumps of coal
are lying about, and in a ditch an actual seam of coal,
whence they were probably derived, is seen in place,
with shales above and below. The outcrop of this
seam is drawn, and the boundary of the sand at the
same time, by traversing the ground, so as to cross
and recross their lines. These are found to gradually
approach each other, so that the coal-seam — or rather
its outcrop — dies away at the boundary of the sand ;
the coal, with its associated shales, passing beneath the
red sandstones.

A little further east the edge of the sand suddenly
appears to trend nearly southward, running indeed
almost at right angles to its direction as far as this
point. The exact position of the line here is not very
distinct, so it is drawn somewhat doubtfully, when at
about five chains to the south it is found to resume its
original direction. There is evidently some break in
the continuity of the line here, and it will have to be
ascertained whether this is due to a fault, as is pro-
bably the case, judging from the apparent shift in
outcrop.

From this point the sandstone line nearly follows
the contour, almost to the edge of the map, where
another coal-seam abuts against and passes beneath
its boundary. These lines of outcrop of the coal-seams,
abutting as they do against the winding boundary of
the sandstone, testify to a difference in the line of

PROCEDURE m THE FIELD. 83

strike, and a consequent difference in the direction of
dip of the two formations. It will be at once apparent,
"without an actual section showing the beds in their
relative position having been met with, that the fact
observed is a positive proof of the existence of either a
fault here also or an unconformity. The second coal-crop
is mapped in another walk to the westward, and is found
to be broken in a manner similar to that of the sand-
stone boundary. Another breakage occurs in it further
on ; however, the outcrop is mapped, for the present,
as nearly as may be from the evidence obtainable, and
the surveyor proceeds to the other side of the stream,
having first drawn a line corresponding to the northern
margin of the alluvium.

In following the alluvium line on its south side he
meets with a wall-like mass of rock standing well up
a,bove the surface of the shales which form a flat in
the bottom of the valley, and cutting across their
strike in a south-easterly direction. A detached speci-
men of this rock can be scratched only with difliculty,
and it does not effervesce with acid, therefore it is a
silicate ; being grey in colour, weathered white on its
outer coating (which effervesces along its inner
margin), and compact in texture, it is almost certainly
Felsite, the mass being either eruptive or intrusive.
It proves, when afterwards mapped, by the change it
makes in the shape of the ground, to be not inter-
bedded with the coal and shales, but an eruptive dyke
breaking through the strata without the slightest con-
formity to their planes of stratification.

The next easterly traverse, after the completion of
the southern edge of the alluvium, crosses another coal-

6—2

84 ENGINEERING GEOLOGY.

seam^ and the edge of some red sands and marls
similar to those on the north side of the stream. A
small section in the shales associated with the coal
shows the dip to be S. 8** ; the sands where exposed
in a deep ditch are seen to be dipping also in a
southerly direction, at an angle of 6° only — an
additional proof of the unconformity between them-
In a small pit further east the dip of the sands has
increased to 8** 30' ; and it must be observed that this
is in an exactly opposite direction to the dip obtained
in quarry No. 1, showing that the beds form an anti-
clinal arch, its axis coinciding with that of the little-
valley. But in the neighbourhood of these sections a
break occurs in both the coal-crop and the sandstono
boundary, similar to the breaks on the other side of
the stream ; and it is seen, when the lines are mapped,,
that another nearly straight line would pass, with one
exception, through all the points where the boundaries
and outcrops are broken. This fact testifies to the
strata beiug dislocated by a ^fault,^ of which the
sudden change in dip previously observed is also an
indication. A line drawn through all the fractured
ends of the beds represents the line of the fault ; on
its western side the beds have been thrown down to an
extent that will afterwards be ascertained. A second
fault, nearly at right angles to the first, accounts for
the break in one of the coal-seams somewhat to the
westward of the other fractured portions.

The boundary of the southern portion of the sand-
stone is laid down in this traverse, with two outcrops
of coal-seams and one of a thin band of limestone, all
following the physical feature, or shape of the ground.

PROCEDURE IN THE FIELD. 85

The limestone is found to be cut across by the felsite
dyke, of whicli the mapping is completed by carefully
following its irregular edge, the line being drawn as it
is discovered, for it has no relation whatever to the
general contour. (See page 83.)

In the next walk to the westward, another set of
beds is found coming on above the red sands and
marls, noticeable at first by the great change in the
-colour of the soil, and by lumps of limestone scattered
over the higher portion. A section will perhaps be
found in these new beds; but in the meantime the
boundary can be readily traced by ditches, the differ-
ences in soil, and similar surface indications. About
half-way across the map another quarry is met with
in the sandstone, and the exposed section is thus

noted : —

Quarry No. 2.

Alternating and varying beds of mica-
ceous sandstone and sand, indistinctly
bedded, of reddish colour, with shaly
divisions and patches of gypsum . 30 feet

. Dip S. 10\

The uppermost bed in this section is a rubbly
mass of calcareous shale and weathered pieces of lime-
stone, probably remnants of a deposit once overlying
the sandstones, but removed by denudation.

The face of the quarry on the south shows the beds
in a horizontal position ; the dip observed on one side
is therefore correct, in amount and direction.

In returning eastward, and not far from the comer
of the map, a large quarry is met with in the beds
mentioned as probably coming on above the red sand-

86 ENGINEERING GEOLOGY.

stones and marls. It is worked for building purposes,
the material being of excellent quality, and, although
easily worked when first quarried, it is compact and
durable : the stone can be easily scratched by a
knife, it effervesces slowly with acid, is compact and
of grey colour, and proves to be Magnesian Limestone.
The face of the quarry is very irregular, but a project-
ing angle at one part gives an opportunity of takings
two apparent dips with their direction. These and
their result, with the notes on the beds, are given
below : —

Quarry No. 3.

Magnesian limestone, tabular and
jointed, in beds from two to four feet
thick, with thin earthy layers between 40 feet

The upper beds are light-grey in colour,
with many fossils and some concre-
tions. Lower down the stone gets
harder, and in some cases granular.
The lowest beds are of darker colour,
more gritty texture, and, in some parts,
distinctly crystalline.

Apparent dip to S.S.B, 13°) __qi -.j^o

„ W.S.W,5°J"" •

Judging from the observed dips, and the position of
Quarry No. 2 below that of No. 3, the red sandstones
noted in Quarry No. 2, and similarly in No. 1, must
pass in under the Magnesian Limestones, between it
and the Coal Measures. They probably are therefore
the sandstones and marls of Lower Permian, as the
limestones are of Middle Permian age.

PROCEDURE IN THE FIELD.

GLAcrAL Drift.
Boulder Clay, 100'. ,

Middle Permian.
Magnesiau Lime-
stone, 360'.

Lower Permian

Sandstone and

marls, 280'.

•1

Upper

Carboniferous.

Coal Measures,

900'.

Millstone Grit.

*« • « • • ^

I • tt » o.

3CX

100 feet Brown stony clay.

360 „ Hard limestone rock, ris^
ing N. 17°.

280 „ Bed sandstones and
marls with gypsum.
Much water.

60 „ Shales.
3 „ ♦ Three foot' coal (3' 8"), rising N.

113 „ Shales.

8 „ ^ Hard blue limestone rock.

156 „ Shales.

li „ *ThincoaP(l'6'0,risingN.
317 „ Shales.

3 „ * Lower Three foot ' coal (30,LN.

l:7Gi „ Shales.
4 „ Fire clay.
78 „ Shales.

1 „ * Bottom coal ' (1' 8"), rising N,

80 „ Shales.

t — —1 Gritstone (Millstone
Grit).

Fig. 5.— Section of Beds passed through in sinking the Upcast

shaft at Colliery.

88 ENGINEERING GEOLOGY.

The Magnesian Limestones are found to be covered,
at the south-east comer of the map, by a brown sandy-
clay, unstratified and full of fragments of rock o^
various kinds, and — ^in this respect like the felsite
dyke — its boundary fails to follow the shape of the
ground. This clay is known as ' Boulder Clay,' and
is one of the members of the ' Glacial Drift,' the
boundaries of all which deposits, whether clays, sands,
or gravels, are most irregular, as explained on
p. 89. Being closely followed, this irregular deposit
is found to fall rapidly away to the north, and evi-
dently must soon mask the boundary of the lime-
stone, perhaps that of the sandstone also, even if it
does not descend quite into the valley.

In walking over the Boulder Clay in search of
sections, and to complete his traverse of the area, the
surveyor comes to the pit's mouth of a colliery. The
coal-seams down to and through which the shaft has
been sunk, and in which mining is carried on, are no
doubt those which rise to the surface in the lower
ground to the north, and were found to be dipping in
a southerly direction. Figure 5, p. 87, represents a
vertical section, made from particulars kept at the
manager's office, of the strata passed through in sink-
ing the shaft.

Drift deposits. — The different sets of fossils enclosed
in the stratified formations are remains of plants and
animals which flourished respectiv^ely during the
periods of their deposition. These remains — some sets
of which are of a tropical, some of a temperate, others
of an arctic type — prove many important changes in
the climate prevailing in the same area at different

PKOCEDURE IN THE FIELD. 89

periods of the earth's history. During at least a part
of the time included in the Glacial period an arctic
-climate prevailed over Britain, covering the land with
ice and snow. Of this there is ample evidence, also of
concomitant physical conditions, by which the geo-
logical products of that age are strangely affected.
There were then great oscillations in the relative level
of land and sea, so that, in turn, every part of the
surface of these islands was more than once subjected
to the abrading action of moving ice. This agent
denuded the old surface in a rough and irregular
manner, leaving equally rough and irregular masses of
the abraded material.

The deposits of this period are chiefly unstratified
clays, in which are seen numerous rounded and
angular lumps of older- rocks, laminated and some-
times contorted brick-earths, huge moraines of angular
Tock fragments, unstratified gravels, and false-bedded
sands. All these occur in a partial and, in one sense,
an uncertain manner, as regards thickness, extent and
distribution, the only regularity being their succession
{on a large scale) in definite sequence. It matters not,
for practical purposes, what form of ice has produced
these results, whether land-ice, coast-ice, or icebergs,
or whether one only or each of these agents of glacial
action may have had its share in the production of the
deposits which are, however, admitted by all to have
had a glacial origin.* As the clays and gravels occur
in the irregular manner described, their boundaries
will not have the same mathematical relation to the

* The views of the author on this subject are expressed in a
Survey Memoir on * The Geology of Cambridgeshire.'

90 ENGINEERING GEOLOGY.

shape of the ground as those of stratified beds. In
this respect they are analgous to those of the eruptive
rocks (p. 84), and are traced in a similar manner by
being closely followed. Evidence of the lines must be
obtained at frequent intervals, without trusting to
contour or stratigraphical continuity, if any scientific
or practical conclusions are to be drawn from the ex-
tent of the beds or their mode of occurrence.

The climate has, since the Glacial period, gradually
ameliorated, and as the land arose from its last sub-
mergence (a fact also generally conceded) spreads of
gravelly material were formed, here and there, upon
the surface, derived from the waste of the glacial
deposits or of older rocks as the case may be. These,
now reduced to mere remnants, are found usually on
the highest lands and in patches of no great extent.

Next in order are some sands and gravels, also
occupying high lands as well as ridges within valleys,
which indicate the earlier positions of the rivers. In
some instances the old courses may have been coinci-
i. dent with those which the rivers now occupy at a
lower level, bub were usually distant from them ;
sometimes parallel with, but more frequently trans-
verse to, the lines of the modern streams. At lower
and lower elevations, are found other terraces of river
gravel, each marking stages in the excavation of the
valleys in which they occur, until those bordering the
rivers are met with at a level just above them, which
belong to the most recent stage of all before the
rivers scooped out their present channels.

As a rule, these old gravels, which contain prehistoric human
reUcs and the remains of extinct mammalia, are not continuous,

PROCEDURE IN THE FIELD. 91

but occur in lines of small and usually elongated patches,
capping mounds and ridges, and more rarely banked against
the flanks of existing hills. It is evident that they were not
deposited in such positions, so far, at least, as the mounds and
ridges are concerned, but that the line of ground they cover
was a valley at the time of their deposition. The patches,
therefore, rest in old hollows or depressions, which give to their
lower layers a synclinal form that has, in a great measure, con-
tributed to their preservation, while the surrounding ground,,
formerly higher than that which they occupy, has been removed
by denuding agencies. This trough-like form of section in thes&
gravels ha^ a bearing upon their water-yielding capacity ; all
the water within them, at a lower level than their boundary, is
retained, but it would otherwise be thrown out along the^
margin, and it forms the source of supply to the surface-springs
upon which many of the smaller towns and villages depend.
('Field Geology,' p. 312.)

The lines of these deposits are easily drawn ; they
follow the contour, or nearly so, from any point where
the boundary may be discovered, except where the
beds rest against the flank of a hill, and do not form
a ridge or mound of their own. In these cases allow-
ance must be made for the bed thinning out against
the slope of the hill, and in drawing all boundaries of
sand or gravel the area enclosed by the lines should
not spread as far outwards as the surface-soil would
indicate its extension. The line must be kept up, or-
back, to allow, according to the probability in each
instance, for the material which will have been washed
down the sloping ground and thus gives a deceptive^
appearance of the bed to some distance below its
actual boundary. *

CHAPTER V.

METHODS EMPLOYED IN GEOLOGICAL SURVEYING

{continited) .

Geological sections — Practical value — Levels -Filling in.

4Jeological Sections. — The amount of information fur-
nished by a geological map is greatly increased if the
map be illustrated by a section showing the under-
ground extension of the rocks occurring within the
:area portrayed. When a geological map has been
properly made, it conveys a valuable but still limited
knowledge of the sequence of the beds, and if the
works for which such knowledge is required be affected
•only by their surface characteristics a section is un-
necessary. But for all purposes' in which the more
deep-seated rocks and their relation to each other, and
to those near the surface, would exercise an influence,
a section is invaluable. This may be either a simple
diagram or a section drawn to scale : for the latter a
line of levels is of course requisite, showing with
«,ccuracy the form of the surface along the route to be
thus illustrated. The geological details may be filled
in either from surface evidence and details of actual
•exposures or borings along the line, or from surface
evidence, physical features, and scientific deductions.
Diagrams are useful, whether exaggerations or not.

PBOCEDURE IN THE FIELD. 95

to illustrate any particular points, as by their aid the
mind is enabled to grasp details much more readily
than from verbal description alone. Sections drawn
entirely from observed facts are of course the most
trustworthy, but it rarely happens that any number of
exposures, or borings of any depth, can be obtained in
the line of country which the section is intended to
traverse. Therefore geological sections are generally
made from evidence obtained at the surface, all the
available data being studiously collected. The dips
are worked out from these, all the known natural phe-
nomena bearing upon the hidden rocks of the district
are carefully considered, and the probabilities of other
and more deep-seated influences upon them are taken
into consideration. From the results thus obtained,
supplemented and checked, perhaps, by an occasional
well or boring, good geological sections can usually be
drawn. It may be confidently asserted that if due
attention has been paid to the points indicated, such
sections may be relied on as presenting, if not an
accurate representation, certainly an approach to the
facts as they occur in nature, sufficiently near even for
all practical operations.

Practical Value of Geological Sections. — The know-
ledge afforded by geological sections of the position of
the various rocks a long way beneath the surface of the
earth is (as urged on pp. 44 — 47) especially valuable
to the engineer in laying out his lines for tunnels
and deep cuttings. It may often be desirable to
follow the course of a particular bed, or series of beds,
on account of its value as building material or ballast,
or of the facility with which it can be penetrated, or

94 ENGINEERING GEOLOGY.

because it would form, owing to its strength or con-
tinuity, a good roof for tunnelKng operations. On the
other hand, it may be equally desirable to avoid certain
rocks, on account of their bad qualities in one or all of
these respects, or because of their liability to slips, or
from the probability of their yielding a troublesome
flow of water; but unless the internal structure of the
hill be known, the desired end is almost certain not to
be attained.

In designing such works to the best advantage, a
point would be selected for the commencement of the
tunnel or cutting where the most suitable beds come
to the surface at the proper level for the construction
of the line, as determined by the equalisation of earth-
work. Then the strike (not necessarily the local out-
crop) must, if possible, be followed, a course which
would insure the continuance of the work through
those beds so long as the base of the cutting or tunnel
should remain at the same level. Some amount of
fall, in one direction or in both, would, however,
generally be necessary, and by swerving a little to
one side of the line of strike — that is, to the rise of
the beds, according to the amount of dip and requisite
fall — the same result of continuance in the same beds
would be insured. Not only would the work be of
similar nature throughout, but also the resulting
material, whilst the risks involved in a change of
strata in tunnelling would be avoided — indeed, the
desirability of thus selecting a line is so great that it
is scarcely possible to over-estimate its importance.

Of equal value is such knowledge in the estimation
^of water-supply from deep-seated springs, for upon

PROCEDURE IN THE FIELD. 95

the relative position and permeability of the rocks far
beneath the surface depends the success or failure of
the projected operations. The artesian-well engineer
who can work out these problems is able to select the
most promising position for a boring, to calculate
approximately the depth to which it must be carried,
and to estimate the quantity and quality of the supply
to be obtained. He can also make his preparations
beforehand for passing through those springs which
are to be rejected, and he knows when and where all
expenditure in boring would be useless for the purpose
in view. In Part III. this subject is more fully dis-
cussed, and an example is given of the methods of
procedure and calculation.

Levels. — It is necessary to have the surface of the
ground along a line of geological section accurately
represented, otherwise errors will creep in where per-
haps they are least expected. The means by which a
line can be drawn to . represent the surface are
sufficiently numerous, and need not here be described ;
by some approximately, by others every rise and fall,
however slight, is shown with the greatest accuracy.
There are the precise methods adopted for engineering
works, in which the level and theodolite are used ; and
others, less accurate, which are however very service-
able on account of their quickness or the portability of
the instruments employed. These are frequently of
much use in unravelling the intricacies of a difficult
piece of geological mapping as the work proceeds, and
the necessary observations can be taken by the sur-
veyor while engaged in traversing the country for that
purpose. Sections are thus drawn from contour maps.

96 ENGINEERING GEOLOGY.

from the Ordnance 'heights above the sea,' or from
observations taken with an aneroid barometer, care being
taken to eliminate the error arising from change of
atmospheric pressure during the time of making the
observations.*

All sections, by whatever mode the data are obtained,
require that heights be known at certain definite
points, varying in distance from each other according
to circumstances, the intervening portions being
sketched in to correspond with the surface configura-
tion. For diagrammatic sections the distances apart
of these points can be scaled from the map, but
for accurate work measurements by the chain are
required.

It is usual in geological sections to refer all heights
to the Ordnance datum, or ' level of the sea,' which is
the level of mean tide at Liverpool. In the sections
published by the Government Geological Survey a
datum is assumed 1000 feet below the level of the sea.
All the points on the Ordnance maps, where heights
are figured, are bench-marks, the levels of each
having been ascertained with reference to the Ord-
nance datum.

As a general rule, the rocks will have been mapped
before the levels are taken for a section to illustrate
their mutual relations, therefore the section line can be
laid out to correspond, as nearly as may be, with the
dip's direction. But sometimes it is found convenient
to construct a geological section along a line which

* The methods usually adopted are described, with examples
of levelling by aneroid barometer, in * Field Geology' (Bailliere),
1879, p. 128.

PKOCEDDKE IN THE FIELD. 97

does not ran In the direction of the dip, when the
beds mnst be shown inclined at a. smaller angle in
proportion to such divergence, the difierence being
found by diagram or by calculation.

Filling m Sections. — For filling in the geological
details to sections, the points of boundary and outcrop
crossed by them are either scaled off from the map or
plotted from notes of their position in the measure-
ments taken in running the surface levels. The beds
are drawn inwards, from these points, with the proper
inclination along the line of section, true or apparent
dip as the case may be, and with all the faults,

Fig 6— Section showmg tha underground extension of the
rocks which come to the ourface in the area aurveyed, as
represented in the geolo^cal map, Fig. 4, p. 80.

flexures, and contortions that can be ascertained ;
dotted lines should be used where these details are
uncertain.

The dip of all the beds in the section, fig. 6, is

necessarily shown much less than it would be if the

line of the section coincided with that of the dip, from

which it deviates 45°, for the purpose of crossing the

98 ENGINEERING GEOLOGY.

fault and of utilising the evidence afforded by the shaft
of the coal-mine. The dip of the limestone^ for instance^
is at its boundary (calculated from the observations in
the quarries) about 10° 30'; it is drawn 7' 30' only, as
the result of its 45" variation from the true dip's
direction. The Coal measures are dipping at 22% and
are shown less in proportion. This difference in
amount of dip, taken in actual section through the
two formations, proves the unconformity inferred from
the gradual approach of their lines of boundary, (See
page 82). The amount of ' downthrow ' of the fault is
calculated from the known dips and the lateral shift
of the beds ; that is, by finding from these the differ-
ence in depth below the surface of the same bed on
either side of the fault where crossed by the line of
section.

PART III.

ECONOMICS— MATERIALS, MINERALS, AND METALS ;
SPRINGS AND WATER SUPPLY.

CHAPTER I.

MATESIALSy MINERALS^ AND METALS.

Economic products of the Eecent and Tertiary Bocks.

The following brief notes on materials, minerals, and
metals are given as affording suggestive indications of
what may reasonably be expected to occur witHn the
limits of the different formations distinguished by
colours and symbols on geological maps. Those who
are engaged in designing or executing engineering
works are recommended to ascertain from good maps
of this kind what formations occur within the area with
which they are immediately concerned, and to trace
out for themselves, according to the directions given
at page 54 et seq. : the local nature and exact extent of
the deposits. These vary in character and economical
value too much in different areas for any general
description to be otherwise of any great practical
utility.

Beyond a doubt there are many beds of good
material, or of other commercial value, and many pro-

1—2

100 ENGINEERING GEOLOGY.

lific mineral veins, the existence of which has hitherto
escaped notice. Others have been merely indicated
on maps and in works primarily prepared for scientific
durposes ; and their extent or value can only be ascer-
tained by strictly local and detailed investigation.
For example, there are numerous bands of ironstone
and of phosphatio nodules, much too thin or discon-
nected to make an appreciable or ' mappable ' outcrop,
which consequently do not appear even on the official
geological maps, and which receive only casual mention
in the scientific memoirs

The following table shows the general sequence of
the geological formations, and the nature of the pre-
vailing rocks in each; their economic characteristics
being more fully described under the headings of the
group to which they belong.

TABLE OF STRATA.

Recent.

Alluviumi SUt, Peat, Blown Sand, Shingle.
Valley Gravels^ sand, hick-earth.
Terrace Gravels {Paloiolithic implements).
Plateau Gravels and ham

Qlaoial.

Boulder Clay. Till Moraines.

Sands, Graxds and loam^

JSriek-earth.

Ckomeb, and elsewhere. — Submerged Forest Beds.

Tertiary.

East Anglia. — Crags. Shelly gravels.
Devonshire. — Bovey Beds. Lignite and days.
I. OF Wight. — Fluvio-marine series. Clays, marlsy sandstones
and limestones.

ECONOMICS. * 101

London Basin.— Bagshot Beds. London Clay. Lower Lon-
don Tertiaries. SaTids^ pebble bedSf claps and variegated
loams.

Cretaceous.

Upper Chalk, with flints. Lower Chalk, mostly without

flints. Chalk, sandy limestones, flints.
Upper Greensand. Gault. Lower Greensand. Sand, sand-

stones, flrestoneSf limestones, clays and ^ coprolites,^
Weald of Kent and Sussex.— Wealden Beds. Clays, sands,

sandstones, limestones and irmistone.

Oolitic.

Farbeck and Portland Beds. Kimeridge Clay. Coral Bag.
Oxford Clay. Corn-brash. Great Oolite. Inferior Oolite.
Limestones, clays, sandstones, grits, flags and ironstone,

LlASSIO.

Upper Lias, Middle Lias or Marlstone, Lower Lias, Ehsetic
Beds. Clays, limestones and ironstone,

Tbiassic.

Keiiper, marls, sandstones, gypsum and rock-salt
Bunter, sandstones and pebble-beds,

Pekmian.
Magnesian Limestones, sandstones and marls.

Cabbonifebous.

Upper Carboniferous. Coal, shales, sandstones, grits and iron

stone.
Lower Carboniferous. Limestones, sandstones, days, Trappean

rocks, ores of iron, lead, zinc, etc.

Devonian and Old Bed Sandstone.

Sandstones, slates, marls and iron-ore. Granitic and Trappean
rocks.

102 ENGINEEHING GEOLOGY.

Silurian.
Sandstones, gi'tts, flags, shales and limestones.

Cambrian.

Slates, sandstones, flags, Granitic and Trappean rocks, ores of
copper, lead, zinc, etc,

Latjrentian.
Granitic and metamorpkic rocks, sandstones, etc.

Great care is necessary in the selection of building
materials, especially of those required for important
public works, where strength and durability are indis-
pensable qualifications. In the selection of a stone,
it is not merely in the testing it by hardness, composi-
tion, and appearance, that judgment should be dis-
played, but also in ascertaining the conditions under
which it lay before removal from its position in the
quarry. For instance, many of the oolitic limestones,
which are hard and compact, appear to the eye as
excellent building stones, but spUt up into thin plates
and fragments on exposure to the weather, from the
effects of which they have been hitherto preserved by
several feet of superincumbent clay.

A method, adopted by M. Brard, of testing the
relative value of various stones as building material, in
regard to durability, especially applicable to oolitic
and other calcareous rocks, is to be found at page 464
of the second edition of Ansted's ' Geology.' The
action of frost is induced by the crystallisation of
Glauber's salts in a saturated solution of which cubes
of the stone are boiled; they are then suspended

ECONOMICS. 103

over the solution, and plunged into it for the removal
of efflorescent crystal, during a period of four days.
The particles scaled oS by the process are then
weighed^ and form a comparative index of the dura-
bility of the stones under examination.

Another point to be borne in mind is that some
finely laminated sandstones and shelly limestones
having a similar structure will weather off in flakes if
the stone be used as flags for pavings or be placed
with its lines of laminae vertical in walls^ or parallel
with any external face of a structure. With their
edges only exposed to the weather these stones last as
long as any other, and in this way are frequently used
as coping stones to rough walling. Again^ 'Sand-
stones being composed of siliceous grains^ they are
more or less durable according to the nature of the
cementing ^ubstance^ while limestones are durable in
proportion rather to the extent in which they are crys-
talline^ {Ansted). But the best and most reliable
test of all^ so far as durability is concerned, is for
the engineer or architect to examine for himself,
old structures known to have been built from certain
definite beds taken from quarries in the neighbour-
hood.

Professor Hull, in his treatise ' On Building and Ornamental
Stones/ says that ' the density of a building material i^ to a
certcdn extent a test of its compactness and durability.' Also
that ' in selecting a stone special attention should be paid to
the climate of the locality in which the building is to be
erected. The chemical constituents of rocks are of great im-
I>ortance in reference to their durability under certain circum-
stances of the atmosphere, and conditions of climate. The

104 ^ ENGINEERING GEOLOGY.

presence of smoke, of sulphurous, hydrochloric, and other
acids powerfully aids the destructive effect of rain or moisture ;
limestones and dolomites are especially subject to disintegra-
tion from the influence of rain charged with acid. The best
kinds of building stone for smoky and wet climates are sili-
ceous sandstones, formed of grains of quartz, cemented to-
gether by a siliceous or felspathic paste.'

The stone whicli is most suitable for building pur-
poses is rendered, by the same properties, the best also
for ballasting lines of railway; that which falls to
pieces by the action of the weather must, of course, be
scrupulously avoided. The stone employed for road
metal should be tough as well as hard and durable,
and, if possessing an uneven texture, it is to be pre-
ferred on that account. For this purpose ' sandstone
is better than limestone, and hard limestone better
than slate ; while basalts and granites are exceedingly
good or exceedingly bad, according to the proportion
of alkaline earths which they contain ' (Ansted).

In the following notes upon the occurrence of build-
ing materials amongst the various geological forma-
tions, the rocks of igneous origin are mentioned in
connection with the products of the sedimentary
deposits with which they are associated, their geo-
logical age being unimportant for practical purposes.

Eecent Deposits.

Alluviunt and River Gravels. — The materials fur-
nished by the alluvial deposits of existing rivers and
streams are comparatively few and unimportant.
Consisting mainly of muddy loam and silt, the allu-

ECONOMICS. 105-

vium makes excellent pastare land^ but yields no
building material beyond ah occasional bed of brick-
earth, sometimes worked to a depth of a few feet in
the absence of more suitable deposits. The river silt,
when coarse, sharp, and clean, like the ' above-bridge
sand ^ of the Thames, is valuable for building purposes,,
especially for stucco and similar work ; when fine in
grain, this sand is useful also, in the process of brick-
making.

Deposits of brick-earth occur here and there at a
level slightly above that of the present rivers, and are
sometimes of excellent quality, as well as of consider-
able extent and thickness ; such are the beds exten-
sively worked on the banks of the Thames below
London. The gravels of this series are generally fino>
the stones of which they are made up being usually
small and subangular; the material they afford is
used for road-mending and ballasting railways. A
greater part of the metropolis stands on gravel of this
age and description. Bath-bricks are manufactured
from a recent sand full of siliceous remains of minute
organisms, and terra-cotta has been made from an
alluvial clay in Devonshire. There are other deposits
of gravel similar to the last, but usually coarser, which
occur as terraces, at higher levels, and mark the
former course of existing or of ancient rivers. These
gravels are sometimes remarkable for their enclosing
rude flint implements, relics, so far as can at present
be positively asserted, of the earliest inhabitants of
these islands. (See page 90.)

106 engineering geology.

Glacial Deposits.

It was mentioned in Part II. that the surface of
many parts of the country is covered by drift deposits,
^nd that these occur alike on hills and in valleys, in a
very irregular manner. As a rule they have no rela-
tion to the present form of the ground, but mask the
older rocks over some large areas, and eCre at the same
time totally absent from others. The accumulations of
this period consist of clays, gravels, sands, and hrick-
€arths, varying in composition and appearance, but all
containing fragments of the older rocks.

The clays are occasionally used for brick-making, it
being essential, however, that they are worked up in a
pug-mill to get rid of the included fragments ; in some
instances they are so calcareous as to be burnt for
lime. The chalky boulder clay has been much used
in former times for marling land, and with excellent
results. The brick-earths form extensive beds, and
«,re largely worked in the Eastern counties ; from one
of these deposits the famed Woolpit bricks are manu-
factured. There are many large but irregular spreads
of glacial gravels and sands, varying in coarseness,
the former yielding good material for making roads
and ballasting railways, the latter for building and
brick-making. Some of the gravels are made up
more or less of rounded pebbles, and afford useful
stones for paving and ornamental building purposes.

The gravels and sands of this and the preceding
series make light soils which require high farming,
their yield being also greatly dependent upon the wet-
ness and dryness of the seasons. The brick-earths

ECONOMICS. 107

form loamy fldls, espedaUj valuable for hop-growing
and for market-gardening; tlie boalder clay makes
excellent corn land^ its fertility and ease of working
being greatly increased by early ploughing and ex-
posure to the frosts of winter.

The recent and glacial deposits occur under con-
ditions favourable to the production of surface-springs.
(See p. 133.) But the beds are so irregular, and
sometimes thin out so rapidly, that their occurrence
at any point cannot be depended on without care-
ful examination.

Tertiary Rocks.

The Pliocene deposits are almost entirely confined
to the Eastern counties, and yield but little material
calling for notice here. They consist in great measure
of shelly sands or ' crag/ used for gravelling garden
walks and private roads, with occasional beds of clay
and brick-earth. One notable product, however, of
these deposits is the Suffolk bone-bed, a seam of
' coprolites,' or phosphatic concretionary nodules, con-
taining 45 to 60 per cent, of phosphate of lime, which
has been largely worked as a source of artificial manure,
but is now almost exhausted.

The Mioce}ie beds are rare in this country ; they
include the ' Bovey coal,^ seams of lignite worked near
Bovey Tracey in Devonshire, and some beds of true
coal in the islands off the west coast of Scotland.
Some felspathic clays belonging to the same series,
and of considerable thickness, occur in the same dis-
trict as the Bovey lignites ; they are used for pottery,
and yield pipe-clay of good quality.

108 ENGINEERING GEOLOGY.

The Eocene deposits are more extensive and im-
portant than those of the preceding divisions of the
Tertiary period, and occupy the areas known as the
London and Hampshire basins. They consist of clays,
loams, marls, and sands, with occasional beds of rolled
flint pebbles, and more rarely of fuUer's-earth. Pipe-
clay occurs in some of the beds, and seams of lignite
are not uncommon. The clays and loams are much
worked for brickmaking, especially the mottled loam
of the Woolwich and Reading series, and the well-
known London clay. The latter contains layers of
septaria, or clayey limestones, called cement-stones,
which are dredged off Sheerness, Harwich, and else-
where, for the making of Boman cement. In places
there occurs a calcareous sandstone; ferruginous
sandstone and the many-coloured Alum Bay sands
are found in the Isle of Wight, with a remarkably
pure and white sand employed in the manufacture of
glass.

The Eocene soils are various as the beds themselves,
the Thanet and Woolwich sands forming light land,
intermediate in character between the rich loams of
the Beading beds and the barren sands of Bagshot^
heath and of the same age elsewhere. The London
clay is tenacious, and forms a stiff soil, valuable scs
pasture, and as arable fairly productive when well*
drained ; the marls of the Hampshire basin are of a
fertile character.

Where the Tertiary sands form the surface stratum,
they yield water, but its quality — like that from all culti-
vated lands — should be, in every instance, examined.
The Bagshot sands throw out a line of springs around

ECONOMICS. 109

the lower portion of their boundary, by wjiich it can
be readily followed. The beds below the London
clay formerly contributed greatly to the supply of
wells in and near London, but their yield has been
much reduced by constant pumping, and at the
present time is quite insufficient ; they form, however,
a useful source of supply, where a very large quantity
is not required.

I^IBKAHY

jUNIYKl^SlTV OF

CHAPTER II,

MATERIALS, MINERALS, AND METALS — Continued.

Economic products of the Secondary Rocks.

Upper Cretaceous.

The Chalk is an earthy limestone, consisting of about
ninety-five parts in a hundred of carbonate of lime;
generally white and soft, but containing some hard,
sandy, grey beds, which ate quarried for buUding
purposes. Some of these harder beds, although easily
cut, are well able to resist the action of the weather ;
they have been largely employed for tracery in church
windows, and during the lapse of several centuries have
suffered very little from atmospheric influences.

A bed of hard grey sandy chalk, slightly yellow in
colour, which might well be called a fine-grained
calcareous sandstone, occurs at the top of the Chalk-
marl ; it is known as the^ ' Tottemhoe Stone,' and
has been much used in building. The white chalk is
burnt into lime, the grey chalk into an excellent
hydraulic lime; the softer and more pure beds are
levigated and made into whiting.

The Upper Chalk contains many horizontal layers of
flints, usefulfor building and road-making; in some parts
where building material is scarce, the walls of houses

ECONOMICS. Ill

are built almost entirely of flints, having brick or free-
stone at the angles. In a few old church towers the
necessity for even this addition was dispensed with by
their being built circular on plan. The flints are used
also in the manufacture of china and flint glass. At
the base of the Chalk occurs a bed which in places
abounds in phosphatic nodules; it has been exten-
sively and very profitably worked in many localities
on and near its outcrop in Cambridgeshire, Bucking-
hamshire, and elsewhere. The mineral phosphate, of
which these nodules contains about 55 per cent., is
converted by chemical treatment into biphosphate of
lime, and extensively used as artificial manure.

The Lower Chalk and the Chalk-marl make fertile
soils, much more so than that of the lighter Upper
Chalk, which on its higher and more exposed portions,
such as the North and South Downs, is scarcely
covered by vegetable mould. It, however, produces a
short but constant growth of herbage, on which sheep
are profitably fed, and in some seasons the cultivated
portions will yield fair crops of turnips.

The Chalk is an excellent water-bearing formation,
not only on account of the quantity it will yield under
certain conditions, but also of the quality of its water.
Springs once met with in the Chalk are constant if
not uniform in their yield, owing to its extent, to its
great thickness of rock of fairly equable character,
and to its absorbent power. Chalk absorbs water more
rapidly than any other solid rock, which, however,
passes very slowly through its mass;, in consequence
of this peculiarity the large springs are met with only
in the numerous fissures (forming natural collectings

112 ENGINEERING GEOLOGY.

galleries) by which some parts of the formation are
traversed in every direction.

Although the Chalk is, at or near its outcrop, a
reliable source of water supply, it does not do to
depend on meeting with powerful springs in it, be-
neath the Tertiary strata, except where the water
occurs within it under certain conditions — where
fissures are proved by existing wells to be numerous,
or where the boring is to be carried down below the
line of saturation. (See page 138.) It is an unusual
circumstance, but borings have been carried down
even several hundred feet in the Chalk without tapping
a single spring of any consequence.

The Upper Gh-eensand consists mainly of sands,
and sandstones of varying degrees of hardness, fre-
quently argillaceous, sometimes calcareous, sometimes
micaceous ; the calcareous sandstones are quarried for
building, and occasionally burnt for lime; the more
siliceous beds also furnish excellent building stone.
This formation contains many beds of firestone, notably
in Surrey; the malm-rock, which occurs in Hamp-
shire; many beds of chert, which makes good road
metal ; also phosphatic nodules in sufficient quantity
to be of economic value within the Wealden area.

Exposed in many valleys cut through the chalk, the
Upper Greensand gives rise to a particularly fertile
soil, yielding good returns for the higher systems of
agriculture. Where it occurs in sufficient thickness,
it yields water of good quality, which is held up
by the impervious Gault clay beneath.

The Qaxdt is a persistent formation of stiff blue
clay, calcareous, with occasional bands of septaria.

ECONOMICS. 113

and having, near its base, workable bods of phosphatic
nodules ; being well suited to the purpose, it is dug in
many places for brickmaking. At the base of the
Chalk in Norfolk and Lincolnshire occurs the Hun-
stanton limestone ; neither the Upper Greensand nor
Gault being present, this rock is probably the repre-
sentative of one or both of those formations. Although
good for pasture-land, the Gault soils otherwise are
not generally productive.

Lower Cretaceous.

The Lower Greensand formation varies considerably,
comprising in different localities sandstones, sand,
limestones and clays. The sandstones are mostly
ferruginous, sometimes occurring as coarse grits
quarried for road-mending, and are known locally by
the name of ^ carstones.^ . The sandy beds in the Weald
are termed 'hassock,' and alternate with beds of
Kentish rag, a cherty limestone of bluish-grey colour,
sometimes burnt for lime, and largely employed in the .
construction of important buildings. The clays in-
clude some calcareous bands, seams of phosphatic
nodules, and layers of septaria used for making
Boman cement; the clays are used in the manufacture
of Portland cement. The sands also yield phosphates,
and fuller's earth is not uncommon; there are beds
of white sand used in glass-making, and some very
hard beds have been found suitable for millstones.
In the West of England the Lower Greensand yields
some rich iron ore ; similar deposits have been worked
also in Bedfordshire and elsewhere.

This formation is, from its great permeability, its

114 ENGINEERING GEOLOGY.

continuity and extent of outcrop, reliable as a
source of water-supply; indeed, it was at one time
proposed to supply London by deep borings into the
Lower Greensand. If the series of permeable strata
of which it consists had been proved to exist (as be-
lieved) in absolute continuity beneath the metropolis,
no more constant source of good water need have been
desired. But it is not so ; there is a ridge of older
rocks, buried beneath the Secondaries, standing up
sufficiently high to cut out the Lower Greensand,
which occurs only in thin patches, if at all, beneath
the area where the supply was required. The exist-
ence of this old ridge has been established only by
recent borings, and could not have been suspected
merely from a survey of the outcrop of the Cretaceous
rocks, which is unbroken all around the district
which they occupy. The beds, no doubt, thin out
gradually against this elevation, and where they occur
in any thickness, invariably yield, when pierced, a good
supply of water ; resting, as they do, on the S. side of
the ridge upon the Weald clay, and on the N. upon a
clay of either Oxford or Kimeridge age.

The Wealden series consists of alternating clays and
sands with intercalated bands of limestone, ironstone^
sandstone and conglomerate. The Weald clay is used
for making bricks; near its base are thin bands of
limestone known as ^ Sussex marble,' and it encloses
some layers of nodular clay-iron ore which have been
dug for commercial purposes. There are also beds of
calcareous grit quarried for building and road-mend-
ing; one bed of calcareous sandstone, being very
fissile, is employed for roofing and paving. The beds

ECONOMICS. 115

include some coarse and friable sandstones^ not of a
darable nature, some hard calcareous and ferruginous
sandstone used for road metal, beds of shelly lime-
stone and conglomerate used for roads and for rough
building purposes. These beds also enclose layers of
ironstone which have been worked for smelting.

The Lower Cretaceous soils vary rapidly according
to the change from clays to limestones or sands, some
of them yielding a soil peculiarly adapted to hop-
growing ; the clays are wet and more adapted to graz-
ing, the lighter soils to agriculture.

The water-yielding properties of the Wealden beds
are uncertain, on account of their frequent variation
and lithological peculiarities. Water can be obtained
almost everywhere within the area occupied, but it is
at a considerable depth in many places, owing to the
thickness of the Weald clay — ^this deposit has, how-
ever, some subordinate water-bearing beds. The Weald
is especially an area in which, prior to commencing
borings for water, the local geological structure should
be subjected to thorough investigation.

The following remarks occur in connection with
water from deep wells in Cretaceous rocks, in the
'Eeport of the Rivers Pollution Commission, 1868,*
pp. 99, 102.

* Both with regard to the quality and quantity of the deep
well waters which they yield, these formations (the Greensand
and the Wealden) are considerably inferior to the New Red
Sandstone, Oolite and Chalk. The Chalk constitutes mag-
nificent underground reservoirs in which vast -volumes of water
are not only rendered and kept pure, but stored and preserved
at a uniform temperature of about lO'' C. (50° F.) so as to be
cool and refreshing in summer, and far removed from the

8—2

116 ENGINEERING GEOLOGY.

freezing point in winter. It would probably be impossible to
devise, even regardless of expense, any artificial arrangement
for the storage of water, that could secure more favourable con-
ditions than those naturally and gratuitously afforded by the
Chalk, and there is reason to believe the more this stratum is
drawn upon for its abundant and excellent water, the better
will its qualities as a storage medium become. Every 1,000,000
gallons of water abstracted from the chalk, carries with it in
solution on an average 1 J tons of the chalk through which it
has percolated, and thus makes room for an additional volume
of about 110 gallons of water. The porosity or sponginess of
the Chalk must therefore go on augmenting, and the yield from
wells judiciously sunk ought, within certain limits, to increase
with their age.

* The only drawback to these waters is their hardness, but this
disadvantage is greatly reduced by the circumstance that it is
chiefly of the " temporary " kind, and can be therefore easily
and cheaply removed.'

Upper Oolites.

The Purbeck Beds form a series of clays and lime-
stones, the most important being the Purbeck Marble,
wliichL occurs in thin beds as a compact grey lime-
stone, made up almost entirely of univalve shells, and
formerly much used in the internal decorative work of
churches. A softer limestone, which is capable of re-
sisting the action of fire, is found in these beds, and is
known as the 'burr-stone/ some quarries yield thin
slabs suitable for rooJBng purposes.

The Portland Beds include the well-known Portland
stone, of which St. Paulas Cathedral is built, and
which is largely used for stone stairs and other internal
domestic work. It is a white somewhat oolitic lime-
stone, enclosing shells, and it occurs in beds ranging
even up to 15 feet in thickness, but varying in hardness

ECONOMICS. 117

according to locality; some of the softest beds are
used for holystone. Beds of phosphatic nodules occur,
and are worked in some localities where the Portland
beds are represented. These strata yield a poor and
brashv soil.

The Kimeridge clay is a bluish- coloured shaly clay
with occasional beds of bituminous shale. The clay is
more or less calcareous, and is sometimes used in brick-
making ; it contains layers of septaria, and argillaceous
iron ore has been found near its base. The bituminous
shales have been burnt as fuel and distilled for gas and
mineral oil.

The Purbeck and Portland beds yield a poor and
brashy soil; the Kimeridge clay forms a cold, stiff
and not very productive soil, more useful as grass-
than as arable-land. The Upper Oolites, above the
Kimeridge clay, generally yield water of good quality,
the supply, of course, varying according to the local
conditions; these may be greatly affected, in such
strata, by faults and fissures, especially in the thick-
bedded Portland stone.

Middle Oolites.

The Goral Rag or Coralline Oolite^ occurs as a tubbly
oolite and clay, between the Upper and Lower Cal-
careous Grit, where these members of the same series
are present. It is an earthy calcareous freestone,
sometimes used in building, but is not of a durable
character ; the Calcareous Grits include beds of sand
and sandy limestone. The Coral Rag contains some
beds of oolitic iron ore, which have been worked in
Wiltshire.

118 ENGINEERING GEOLOGY.

The Oxford Clay is calcareous, dark-blue in colour,
and is, in places, largely worked for brickmaking;
it contains much iron pyrites and many bands of
argillaceous limestone nodules; at its base occurs,
over a large extent of country, an irregular calcareous
sandstone, which in some parts is used for building.
It forms a retentive soil, productive in some localities,
but there is risk in its cultivation ; it auswers well as
pasture-land. The Coral Rag and Calcareous Grits
make soils which are light, brashy and far from being
productive.

These beds do not yield much water, except locally,
where the Grits are of a saudy nature, that from the
Oxford Clay (in which water is sometimes found)
being geuerally too impure for domestic purposes.

Lower Oolites.

The Combrash is the only constant member of this
division of the Oolites, a series which assumes a totally
different character, even in adjacent localities, or
rather the beds rapidly thin out, and are replaced
by others. This is a coarse, earthy, blue limestone,
generally weathered, where seen in quarries, to a pale
colour and rubbly condition. It is seldom good
enough for building, but is used as road material, and
occasionally burnt for lime.

The Great Oolite limestones are generally overlaid
by a considerable thickness of clay, referable to the
same period, and, in the South- West of England, also
by the Forest marble, a fissile oolitic limestone, asso-
ciated with thin beds, used as flagstones, for farm
buildings, and roof coverings. The thicker slabs are

ECONOMICS. 119

useful for building rough ashlar work in even courses,
and the material is of good quality for road-mending.
. The Great or Bath Oolite series consists of pale
yellow freestones, finely oolitic and free from fossils,
with bands of shelly and argillaceous limestones. The
Bath-stone is blue in colour far below the surface of
the ground, and is soft when first quarried, but it
hardens and changes to a yellow colour on exposure
to atmospheric influences.

The Stonesfield Slate consists mainly of two beds of
calcareous sandstone, associated with shelly, oolitic,
and sandy limestones. The sandstone beds are so
fissile after exposure to frost that they split up into
' slates,' much used for roofing ; the beds also yield, in
some places, a freestone that is quarried for building.

The Collyweston Slate is a similar calcareous sand-
stone, also largely quarried for roofing purposes.
Intervening, geologically between the two, in Lincoln-
shire, occurs a thick series of marly limestones, known
as the Lincolnshire Oolites, which afford excellent
building stone, and are also burnt for lime.

The Inferior Oolite is darker in colour than the
Great Oolites, and comprises sandy limestones, with
some beds more compact in texture. The freestones
are largely quarried in some localities for building, but
are not generally valuable ; they are soft when first
quarried, and harden on exposure. In Yorkshire is a
series of beds representing the Lower Oolitic period,
and comprising Hmestones and sandstones, used for
building, with beds of oolitic ironstone, which have
been worked for iron ore.

The Northampton Sand is worthy of note as including

120 ENGINEERING GEOLOGY.

a valuable bed of sandy ironstone, often several feet
thick, and which yields 50 per cent, and upwards of
pig iron. The bed as an iron ore is strictly local,
sometimes rapidly diminishing to a few inches only in
thickness, or disappearing altogether. The series
includes also subordinate beds of sandstone, quarried
for building, and clay beds sometimes, although rarely,
pure enough for the manufacture of terra-cotta.

The Combrash is ' said to derive its name from the
facility with which it disintegrates and breaks up —
brashy — for the purposes of corn-land.' — Page, ' As
the name implies, the soil ... is well suited to
the growth of corn. According to Professor Buck-
man, it contains more phosphate of lime than the
subordinate oolitic formations.' — Woodward. The loose
and brashy soils of the Forest Marble and the Great
Oolite limestone are poor and unproductive; those
on the clay are better, but not by any means rich or
fertile. The Lincolnshire Oolites make a light soil,
sometimes red in colour, easily worked, but naturally
not very productive. The brashy soils derived from
the Inferior Oolite is poor on the higher ground, but
in the valleys it is fairly fertile, as are those formed
from the disintegration of the Yorkshire Oolites and
the Northampton Sand.

The Lower Oolites are, from their composition and
structure, well adapted to the distribution of under*
ground waters ; they may generally be relied on as a
source of supply, and in certain favourable positions
they throw out some of the finest overflowing springs
to be found in this country.

ECONOMICS. 121

' Unpolluted spring water from the Oolites is unsurpassed in
its comparative freedom from all kinds of organic impurity.
It is clear, colourless, palatable, and wholesome, and fit for all
household purposes except washing, for which it is too hard.
It may, however, always be softened by Clark's inexpensive
process, and it then unites all the qualities which are most
desirable in water supplied for domestic use. The Oolitic rocks
are very porous, absorbing and holding enormous volumes of
water, which are again delivered as springs, usually of great
size. As water-bearing strata, or as a subterranean reservoir
for the purification and storage of water, the Oolitic rocks are
equal, if not superior, to the Chalk itself. But this vast store
of magnificent water is rarely supplied to communities until it
has been hopelessly fouled in river channels by polluting
matters of the most disgusting description.

' The Oolitic rocks consist almost entirely of carbonate of
lime ; and this substance being soluble in water containing
carbonic acid, springs issuing from the Oolite always contain a
large proportion of solid impurity, of which the most abundant
constituent is carbonate of lime, the remainder consisting
almost entirely of mineral saline matter, also not injurious to
healtL — 'Report of the Rivers Pollution Commission,' p. 120.

Lias.

The Upper Idas is a blue shaly clay enclosing layers
of septaria and nodules of bluish limestone, and in its
lower beds jet is of frequent occurrence. The clay is
much used for brickmaking, some of the beds being
so bituminous that they burn with little or no fuel ;
and the more shaly beds, which contain iron pyrites in
large quantities, for the manufacture of alum.

The Middle Lias, or Marlstone, consists of mica-
ceous finely laminated clays and sands, with marls and
an agillaceous and ferruginous rock-bed, which fre-
quently forms a valuable iron ore. Where the rock-
bed occurs as a limestone it is used for roads and

122 ENGINEERING GEOLOGY.

l)uilding, and is occasionally pure enough to be burnt
for lime.

The Lower Lias consists of clay, with many beds of
blue and grey argillaceous limestones in its lower por-
tion, valuable as building and paving stones, and from
which good hydraulic lime is made. Layers of sep-
taria also occur, which are used in the manufacture of
hydraulic cement; the clays are dug for brick-
making.

The Rhaetic Beds, at the base of the Lias, include
several white limestones, used for lime and building
purposes, ^capped by a hard, smooth-grained stone,
called the *'sun bed,'' which from its closeness of
texture and general purity has been recommended for
the purposes of lithography. At or near the base of
the White Lias is found the Gotham or Landscape
Marble.' — Woodward,

The Upper Lias soils are good as grass-land ; the
Middle Lias beds form a rich soil ' favourable to the
growth of apple trees;' that derived from the rock
bed is frequently red in colour and equally productive.
The soil of the Lower Lias, although brashy in parts,
is fairly fertile, and forms excellent pasture and dairy
land. Some grey marls in this Rhaetic series are
useful for marling land; the soils it makes are of
good quality.

The Marlstone rock-bed yields water, in some locali-
ties only ; as do the limestones of the Lower Lias, but
in quantity not to be relied on, or in quality to be

recommended.

Teias.

The Upper Keitper series consists of variegated

ECONOMICS. 123

marls with occasional beds of sandstone, common
gypsum and alabaster. The marls are dug for brick-
making, the sandstones quarried for building, the
common gypsum is burnt into plaster of Paris, and the
purer kind, or alabaster, is used for ornamental pur-
poses. FuUer^s earth occurs in the marl, and patches
of rock-salt in the marl and sandstone ; very thick beds
of the latter mineral are found in Cheshire and adjoin-
ing counties, and are extensively worked by mining.

The Lower Keilper sandstones, called also water-
stones, form a thick series of micaceous red and
whitish sandstone, with a base, in places, of hard
dolomitic conglomerate. The uppermost beds are
generally finely laminated, those in the centre are good
freestones employed in building. The lower beds
afford road material, and in some places are sufficiently
calcareous to be burnt for lime, in others they yield
copper ore. In the South- West the Triassic Eocks are
magnesian in character, are quarried for lime and for
building purposes, and contain workable beds of iron
ore, also clays which are dug for brickmaking.

The BunteVy or New Red Saiidstone, consists essen-
tially, as its name implies, of red or reddish sand-
stones, sometimes variegated, and of different degrees
of hardness and suitability for practical purposes. At
the base are pebble beds, loose or cemented into com-
glomerate, which in places yield ores of lead and
copper.

The Upper Keiiper marls form a rich soil well suited
for orchards and pasture-land. The lower beds of the
Trias make generally a poor sandy soil.

The Upper Keiiper seldom produces large springs.

124 ENGINEERING GEOLOGY.

and the water is frequently charged with salt ; but the
Lower Sandstones form an excellent water-bearing
formation, being permeable and fairly persistent. The
beds of the Bunter also yield water; indeed the
Triassic rocks, as a whole, are second to none in value
as sources of supply to the deep-seated springs.

* The New Ked Sandstone rock constitutes one of the most
effective filtering media known, and being at the same time a
powerful destroyer of organic matter, the evidence of previous
pollution, in water drawn from deep wells in this rock, may be
safely ignored, unless the previous animal contamination has
been very great indeed.

* The unpolluted waters drawn from deep wells in the New
Eed Sandstone are almost invariably clear, sparkling, and
palatable, and are among the best and most wholesome waters
for domestic supply in Great Britain. They contain as a rule,
but a moderate amount of saline impurity, and either none, or
but the merest traces of organic impurity. The hardness is
usually moderate, and only when the water is derived from
originally impure sources does it become excessive. There is
every reason to believe that a vast quantity of hitherto unutilised
water of most excellent quality is to be had at moderate expense
from this very extensive geological formation.' — *Keport of
Elvers Pollution Commission,' p. 94.

CHAPTER III.

MATEKIALS, MINERALS, AND METALS — Continued.

Economic products of the Palaeozoic Kocks.

Peemian.

The Permian or Magnesian Limestone series consists
of red and white sandstones, magnesian limestone, and
variegated gypseous marls. The chief of these is a
yellowish limestone, composed of nearly equal parts of
carbonate of lime and carbonate of magnesia. Several
varieties of this limestone occur, some being laminated,
others oolitic ; it is generally a good building stone,
but it is important to remember that the beds vary
greatly in durability* Some hard flinty beds are used
for road-mending; the marls have been dug for
making bricks, and in places they contain beds of
gypsum. The red sandstone beds also are much
quarried for building, and the sharper varieties for
whet- and grind-stones ; some of the associated basalts
are largely used as road material.

These beds produce a light soil of reddish colour,
and, on the marl especially, of good quality.

The permeable sandstone beds are water-bearing,
and the limestones also under certain conditions —
which, however, are local, and demand examination.

126 ENGINEERING GEOLOGY.

Carbonipeeous.

The Zfpper Carboniferous series consists of rocks of
greatly varying character ; sandstones, including the
York stone, grits, conglomerates, shales, ironstones,
and coal. The sandstones generally are quarried for
building, the flagstones for paving and, when thin,
for tiling purposes ; some sandstones and grits make
exceptionably good mill- and grind-stones. The clays
are employed, some for making bricks, pottery, and
earthenware, others for fire-bricks and encaustic tiles.
The shales yield iron pyrites, which is used in chemical
manufactures, and the beds of clay-iron-ore are well-
known for their extent and productiveness. This
series includes also the vast and extensively worked
coal measures of Great Britain.

The Lower Garboniferous beds are of more cal-
careous nature, consisting of argillaceous limestones,
crystalline limestones, dolomitic in some localities,
oolitic in others, calcareous sandstones, clays, and
conglomerates. There are some beds of micaceous
sandstone quarried for paving and building. ' The
carboniferous limestome is much quarried for lime. It
is largely used for rough buildings and for road-
mending, for which purpose it is conveyed to great
distances, but from its hardness it is not serviceable
as a freestone. Some of the beds are polished and
used as marble for ornamental purposes.' — Woodward.
Basalts occur, and a fragmentary volcanic ash in
Somersetshire is worked for building material; bor-
dering the Pennine and the Cheviot Hills aro large
masses of basalt and porphyry, and granite occurs in

ECONOMICS. 127

Cornwall and Devon. In the Carboniferous limestone
are many dykes and veins containing ores of zinc and
lead, and many valuable beds of iron ore belong to
this series.

The Upper Carboniferous sandstone soils are barren,
those of the clays and shales are more productive..
The Carboniferous limestone makes a poor ferruginous
soil, in some parts so thin as to be fit only for sheep
pasture.

The water from the Coal measures is not, as a rule^
of good quality, that from the Millstone Grit is better,
and can generally be obtained by boring. The Lpwer
Carboniferous cannot be relied on as water-bearing
strata, especially the Mountain limestone.

Devonian.

This system of rocks comprises many hard sand-
stones, red or grey in colour, with conglomerates,
slaty beds, shales, and occasional limestones. The
slates vary greatly in quality, those quarried in Corn-
wall being the best found in this series; in some
places they pass into useful hone or whet stone.
Greenstone is worked, and basalt occurs in Cornwall
and Devon; also granite, with granitic dykes of a
more felspathio rock, called Blvans, useful as road
metal, aud which forms a durable building material.
The decomposed granite forms kaolin, a valuable china
clay. The Devonian rocks are traversed by veins
which yield ores of silver, copper, lead, zinc, tin,
manganese and iron.

A poor soil is formed by the surface decomposition
of the Devonian rocks, which, owing to their thickness

128 ENGINEERING GEOLOGY.

and permeability, are, in some localities, good water-
bearing strata.

Old Red Sandstone.

As suggested by its name, this series is mainly
arenaceous; it comprises micaceous sandstones, red,
grey and mottled, conglomerates, marly shales and
slates, with local beds of nodular limestone known as
' cornstones.' The sandstones are used in building and
road-mending, the cornstones also for the latter pur-
pose, and the conglomerates for rough millstones ; some
of the beds are firestones.

The Old Red Sandstone yields a loamy fertile soil,
suited to the growth of hops and apple-trees; the
cornstones also make very rich land. The formation
yields large quantities of good water.

Silurian.

The Tipper Silurian rocks consist mainly of hard
siliceous sandstones, sometimes micaceous, grits and
conglomerates, some of the bands being calcareous.
It includes also nodular limestones burnt for lime, thin
shelly limestones, calcareous flagstones, marls, shale
and slates.

The Silurian rocks cover very large areas, and
produce soils, frequently red in colour, of varying
character; the water-bearing characteristics of this
great system of rocks are equally various, according
to locality and physical conditions.

Cambrian.

This group of rocks — which includes those formerly
called Lmver Silicrian — occupies an extensive area and

ECONOMICS. 129

furnislies much valuable building material. Its chief
economic product is roofing slate, varying much in
quality and appearance, and rendering the ground
where that of the best kind occtlrs of enormous value.
The world-renowed quarries of Llanberis and Penrhyn
are notable examples, and the green slates of Bangor,
on the same geological horizon, are much sought for
ornamental purposes, as are those of the same colour,
which occur as part of a higher group in the lake dis-
trict of Westmoreland. The blue and dark slates of
Dolgelly are soft, and those of Skiddaw, although
occasionally used, are unsuitable for roofing purposes,
yielding readily as they do to the action of the
atmosphere.

The^ good building stones abundantly yielded by the
Cafnbrian rocks are sandstones of every degree of fine-
ness and durability, some hard and compact, others
soft and fissile — limestones, varying from impure earthy
beds to fine marble — calcareous slates and flagstones,
siliceous sandstones, freestones, quartzites and con-
glomerates. There are also hard and highly meta-
morphosed grits, sandstones and schists, attaining a
great thickness, and including masses of intrusive
greenstone. Dykes of greenstone occur amidst the
slates of Llanberis and Penrhyn, and near Caernarvon
is an intrusive mass of tough felsite, which is valuable
as material for paving. The granite and syenite of 'a
Charnwood or Chamley Forest are largely quarried .o^
for road metal, and the Whittle-hill oilstones are jslb- S^

tained from the metamorphic rocks of the same ^i^slitjly <\
where the Cambrians occur as an inlier. ^^elfll^^il- ^ ^^
stones come from the Cambrian beds nearj^l^Iclwal^>^^ < v

130 ENGINEERING GEOLOGY.

and Cutlers Greenstone is worked on Snowdon. The
beds yield, at Dinas-Mowddwy, a sand wliicli is used
for lining copper furnaces, and ' some valuable deposits
of phosphate of lime — phosphorite — ^have been dis-
covered on the top of the Bala limestone in North
Wales/ — Woodward. The Cambrian rocks in some
parts yield ores of copper, lead, zinc, iron and cobalt.

Laurentian.

The Laurentian rocks occupy but a very small por-
tion of the surface of these islands, and are generally
included in the same colour on geological maps as
those of the Cambrian system, which they underlie.
They consist of gneiss and gneissose rocks in the
island of Lewis, and of sandstones and conglomerate
in Sutherlandshire.

CHAPTEE IV.

SPRINGS AND WATER-SUPPLY,

Nature of Springs — Surface Springs — Deep-seated Springs —

Water-level.

Nature of Springs, — All sources of water-supply de-
pend entirely upon physical conditions — the rainfall
upon situation and elevation, the rivers upon physical
features, and the springs upon geological characters.
As physical conditions are more or less different in
every district, the sources and available quantity of
water within them also vary, being wholly governed by
those conditions. The physical features have, in ad-
dition, a direct and important bearing upon the methods
of distributing the supply, in the quantity and manner
best suited to the requirements of each locality.

* It may be taken as a general rule that ridges and escarp-
ments consist, wholly or in part, of water-bearing beds, such as
limestone, chalk, or sandstone, and that the softer clays, being
more readily denuded, occupy the lower grounds.

* Strata generally dip towards and pass under the higher
grounds ; it is rarely they occur otherwise, and they are not
often found quite horizontal This is owing to the fact that
the lie of the strata has, in a great measure, given to the ground
its present form ; in other words, anticlinals are more easily
denuded than synclinals, the latter remaining whilst the former
have yielded to denudation.

9—2

132

ENGINEERING GEOLOGY.

Fig. 7.-Section of a ridge and escarpment of pervious and im-
pervious strata, to illustrate the nature of springs and their
water-level.

*Gase 1. — Any set of i?ervious strata which occurs at the sur-
face, as at a a, in figure 7, will (as a rule) form a trough, and
be found below the ridge, B, at a greater depth than is due to
the diflference in height of a and b. Water collected by these
pervious beds upon their outcrop, a a, will rise in a boring
made at b, through the impervious beds, & &, to a height cor-
responding (or nearly so) with that of their outcrop. * Surface
gravels are no exception to this general rule, and often occur
in long hollows on elevated ridges, throwing out intermittent
springs at the lowest points along their margin, d d.

* Case 2. — The same pervious beds, a a, when dipping into
the face of an escarpment, e, probably decrease in dip at a
short distance beneath it, or even become horizontal, and they
would be reached by a boring made on the top of the escarp-
ment, E, at a depth not much below that of their outcrop, the
water from them rising in the bore to an extent coinciding
with the difference, whatever it may be.

* Case 3. — Where pervious beds, c, in considerable thickness,
overlie impervious beds, 6, and form an escarpment, e, or a
ridge, b, the water-level within them rises beneath the higher
grounds until the forces of hydrostatic pressure and frictional
resistance are in equilibrium. Water will, therefore, be
frequently found in borings commencing on a ridge, B, or an
escarpment, e, at a height very considerably above that of the
lower points of their nearest outcrop, 6 6, where the surplus
water flows forth as perennial springs.**

* Extracted from an essay on * National Water-Supply,' by
the author. Joum, Soc. Arts, July, 1879.

ECONOMICS. 133

As the rain falls upon the ground it is at once drawn
by the force of gravitation to lower levels. If the
surface be impervious, the water runs ofE by the ditches,
rivulets, and rivers to the sea ; but if it be wholly or
partly pervious, a portion only thus flows to the ocean,
the remainder passing into the water-bearing forma-
tions. The water which is thus absorbed is again
thrown out, at a lower level, in the form of springs on
the hill-sides, or, the conditions being favourable, it is
retained within the strata at various depths, thus
forming huge subterranean reservoirs. These stores
of water, being supplied from large collecting areas,
and replenished by every shower of rain, are, in the
great majority of instances, almost if not quite inex-
haustible, and not only from the quantity, but also
from the quality of their waters, merit a greater share
than they have hitherto received of practical attention.

Surface Springs, — Although all springs owe their
existence to the same hydrostatic principle, they are
found in many different forms, and under greatly
varying conditions. The simplest kind of spring is
that in which the water occurs over a definite area at
or near the level of the ground, and is, therefore, called
a surface-spring ; it consists of a body of water held
up in a superficial stratum of gravel or sand, by a
bed of clay or other impervious rock beneath. The
water overflows at the lower points along the upper
edge of the impervious bed, thus forming springs
which flow more or less rapidly according to the
seasons, and which, if the collecting ground is small,
may be intermittent.

Where such conditions exist the bed of gravel or

134 ENGINEERING GEOLOGY.

sand is of course saturated up to a certain level, and
any wells dug down thereto meet with land or surface-
springs, which yield a more or less plentiful supply.

But it should be borne in mind that these springs
are fed by rain and surface water which has passed
down through a filtering permeable rock, the thickness
of which is probably insufficient for the elimination of
suspended matter and the oxidation of organic im-
purities ; also that the permeable rock itself is liable
to be charged with the elements of contamination, for
its spring-producing permeability is a great source of
this danger. Imperfect drains and leaking or over-
flowing cesspools within it, farm-yards, and refuse-
heaps upon the surface, must each contribute their
share of pollution, in the form of decaying organic
matter, if not of that which may be of even more
serious nature.*

The supply from surface springs will vary directly
as the seasons ; in periods of drought the streams run
dry, the springs fall ofE, and shallow wells, which
derive their water either by soakage from a stream or
from a so-called land spring, become exhausted in
consequence. Neither streams nor land-springs should
ever be depended on for a supply of water for domestic
purposes, not only on account of their intermittent
nature, but also because of their liability to contamina-
tion.

There are also surface springs, in which the primary
geological conditions are reversed, th^ water-bearing

* See the * Keport of the Rivers Pollution Commission/ 1868 ;
under headings * Shallow Well Waters/ p. 168 ; and * On the
Propagation of Epidemics by Potable Waters/ p. 140.

ECONOMICS. 135

bed of pervious sand or gravel being beneath a thin
covering of impervious clay. In these cases, the
springs of one locality are fed by water from another
at a slightly higher level, but at no great distance, from
which it travels, at a small depth only, beneath the
surface. The water-bearing bed is thoroughly satu-
rated, in this case also, to a level varying with the
seasons, and it yields water wherever penetrated below
that level, but which is also liable to be locally affected
by surface contamination; but it may be that the
higher part only of such a stratum is polluted, for at a
lower part, if at any distance from the upper, some of
the injurious particles may be found to have undergone
a chemical change, and others will be removed by
filtration. Springs of this kind are to be preferred to
those first mentioned ; they may be intermittent, but
are more constant in their flow, and more to be relied
on in respect of purity ; they are intermediate in cha-
racter between the simpler surface springs and those
now to be described, the supply to which passes at a
considerable depth, and from a greater distance.

Deep-seated Springs. — These springs, although pro-
perly called deep-seated, are not necessarily at a great
depth beneath all the area under which they occur, for
their waters will sometimes even flow out again at the
surface, if the beds along which they pass be anywhere
exposed at a level lower than that of the outcrop upon
which they were collected. But they will have been
deep-seated for a greater part of the distance traversed
by their waters, owing to the inequality of the ground,
the dip of the water-bearing beds, or to both causes
combined. And even when they approach or reach the

136 ENGINEERING GEOLOGY.

surface^ after a long underground passage, they differ
from surface-springs in being much more constant in
the yield and equable in the temperature of their
waters, which have been also freed from organic im-
purities by oxidation and j&ltration.

* Surface-polluted water, when it penetrates only to shallow
wells, still retains a considerable proportion of its polluting
organic matter in an unoxidised condition. But when it
descends through 100 feet or upwards of porous soil or rock,
the exhaustive filtration to which it has been subjected, in
passing downwards through so great a thickness of material,
and the rapid oxidation of the dissolved organic matters in a
porous and aerated medium, afTord a considerable guarantee
that all noxious constituents have been removed, even from
such portions of the water as have passed perpendicularly down-
wards. Still more so must this obviously be the case with the
even much larger portion which reaches a well in a more or
less horizontal direction, through far greater thickness of por-
ous medium."

The supply of water to all springs is derived from
the rain which falls upon, or flows ofE impervious strata
to, the outcrop of the pervious strata in which they
occur. The outcrop thus forms their collecting- ground,
and the yield of the springs will be great or small
according to the area occupied by the outcrop of the
permeable beds, the rainfall thereon, its elevation, and
the by no means constant degree of permeability of the
rocks themselves. Other conditions, such as the exis-
tence of &;ults and fissures, and the thinning out of
strata, affect the underground passage of the water,
beneficially or otherwise, and need only to be men-
tioned as worthy of careful consideration in the

* * Beport of the Rivers Pollution Commission,' 1868, p. 89.

ECONOMICS. 137

estimation of water-supply to any locality. The quality
of the water from each spring will be governed by the
chemical composition of the rocks it traverses, and the
readiness with which these yield to the water, by
chemical changes, their organic and inorganic con-
stituents.

As many of the permeable rocks, which form ele-
vated tracts of land receiving their due share of the
rainfall, must in other areas, and at lower levels,
form the floor of the sea, the fresh-water of their
springs is constantly flowing directly into the salt-
waters of the ocean. Such springs are known from
which fresh-water is constantly drawn, and, on the
other hand, sea-water traverses the permeable rocks
and affects their springs, sometimes for many miles
inland. Where the requisite conditions are found, of
pervious beds dipping under the sea and covered by
those which are impervious, fresh-water can always
be obtained by boring to the deep-seated springs.
Springs thus situated form a valuable source of supply
to islands and places along the coast where the water
from the surface- springs would perhaps be quite
useless for drinking purposes, in consequence of its
holding much salt in solution.

For much valuable and detailed information on this subject,
the reader is referred to : —

The Sixth Meport of the Rivers Pollution Commission

(1868), 1874.
Water and Water-Supply. Ansted. (Allen.)
Water Analysis, Professor Frankland (Van Voorst)
Experimental Researches. Professor Frankland. (Van

Voorst)

138 ENGINEERING GEOLOGY.

Water-level, — In surface-springs, the water may be
under va-rious and even under varying conditions, and
the water-level — i.e., the plane of the upper surface of
the water — may be constant or variable according to
the dip of the beds^ the position of the spring, the time
of year, and the variability of the seasons.

The level of the water of a deep spring in pervious
strata, which is kept down in them by overlying strata
that are impervious, as in a, figure 7, will be constant
at, or slightly below, the height of the nearest point
of outcrop of the water-bearing bedis. The water is
under hydrostatic pressure varying with the depfch of
the beds below the level of the outcrop, and its level is
but slightly modified by the frictional resistance offered
to the passage of the water through the more or less
compact material of which the beds are composed.
Consequently the water will rise in a boring made
down to such a spring to the normal water-level ; this
can generally be ascertained, for any spot, by calcula-
tion from data based upon the geological conditions
considered in connection with the physical features.

In deep springs, where the water is held up in a
great thickness of permeable strata by an impermeable
stratum beneath, as in c, figure 7, and the pervious
beds are not saturated throughout their entire thick-
ness, the water-level is influenced by a different set of
conditions in the following manner : — The water-level
of the springs thus formed will be, at the margin of
the pervious bed, coincident with that of their lower
surface boundary, but not merely up to this level will
they be saturated. In the area occupied by their out-
crop, the water-level will be found to rise within them

ECONOMICS. 139

from the marginal line referred to in a ratio propor-
tionate to the permeability of the beds and the amount
of the local rainfall, and to fluctuate according to the
seasons. Rising thus as it does from the lower boun-
dary-line of the permeable beds, it will fall in a trans-
verse direction from the higher ground towards each
lateral valley by which they are intersected. The
reason for this rise in the water-level beneath the
higher ground may be thus explained. The rain-water
which has fallen upon the outcrop of permeable strata
so situated will have percolated downwards, not only
until the saturation was complete to the level of their
boundary, but will have accumulated within them
above that level under the higher ground until it has
reached a height at which the forces of gravity, capil-
larity, and frictional resistance are in equilibrium. It
must necessarily be that the water-level in springs thus
situated will coincide in the valleys with that of the
local streams by which the contiguous beds are drained,
and rise from them in every direction. It is highest
under the loftiest hills, because, with equal slopes,
these would occupy the largest areas, and the water-
level retaining its normal degree of inclination would
attain beneath the centre its greatest elevation.

There may be more than one, even several, deep-
seated springs within a practical depth at any par-
ticular spot, according to the number of alternating
pervious and impervious formations, as, for instance,
in a, and c, figure 7, page 132. Each spring may
have a different water-level, due to the varying heights
of the outcrop of the beds along which their waters are
borne, and to the natural drain upon them by streams,

140 ENGINEERING GEOLOGY.

which lowers it along the courses of the valleys, thus
preventing the normal accumulation-

As an illustration of the phenomena of deep-seated
springs and their water-level, the area included in
Sheet 7 of the Ordnance and Geological Surveys has
been selected. It is coloured as a geological map,
without drift deposits, in the frontispiece, and a section
across a part of the area is represented in Plate ii.
The names of the formations are engraved on the map,
and their water-bearing characters are indicated by
different tints.

The outcrop of the Chalk forms the highest ground,
which forms a large collecting area of permeable beds,
from which a large proportion of the rainfall passes in
under the Tertiary formations towards the south-east.
The Lower Tertiaries are partly pervious, but as the
overlying London Clay is impervious, the water is held
down by them and it, considerably below the level of the
Chalk outcrop. A boring made through the Tertiary
beds, sufiSciently far to meet with a fissure in the Chalk,
would tap its deep springs, and the water would rise
(as in Case 1 or 2, p. 132) to a height nearly coinciding
with that of the outcrop. The water-level would,
however, be subject to this modification : — the area is
traversed by the rivers Thames and Colne, which flow
over the permeable Chalk for a part of its length, and
lower the water-level within it considerably, as shown
in the section. Where this influence ceases the water-
level again rises towards the N.W. (as in Case 3,
p. 132), being highest under the hills, and falling to-
wards all the lateral or tributary valleys.

The section is drawn to an exaggerated scale, but

T. 1 B 11 A 1^ "^

UKlVKHSnV OF

CALIFOUNLV

'V- -..-.

ECONOMICS. 141

the water-level is proved to be as described by several
deep wells along its line ; some of these are appended,*
in the form usually employed for noting sections of deep
wells and borings.

DENHAM.--The Tile House.

Dug 110 feet, the rest bored.
Water rose to 85 feet from surface.

Feet

London Clay. Yellow clay - - - - 22

"o J- T» J ( Gravel and sand, mixed -
Beading Beds. ■{ ^ , , j i. n • j
^ f Gravel, sand and cnalk, mixed

UxBRiDQB.— * The Dolphin.'
Water rose to 3 feet from surface.

- 15

- 30

To Chalk - - 67

Chalk - - - - - 128

Total - - 195

London Clay.
Beading Beds.

Soil, etc. -

Gravel

Clay

Sands and clays -

III*
I I 1 1

Feet

- 4

- 13*

- 20

- 44

Chalk and flints -

To Chalk -

- 81^

- 39^

Total -

121

♦ * Mems. GeoL Survey,' vol. iv. The Geology of the London
Basin, Appendix (Whitaker).

142 ENGINEERING GEOLOGY.

West Drayton. — ^Victoria Oil Mills-

Shaft 12 feet, the rest bored.
Water overflowed.

Feet

Made ground - - - - 3

Valley Drift. Brick-earth and gravel - - - 29

London Clay. Blae clay, sand and pebbles at bottom - 88

Beading Beds. Coloured clays and sands - - - 66

To Chalk - - 186
Chalk 100

Total - - 286

Norwood, near Hanwell.

Shaft through London Clay to sand, 280
feet.

Water rose to the top.

Feet

To Chalk 325

Chalk 89

Total - - 414

EiCHMOND.— ' Star and Garter.'

Water rose to 39 feet from surface.

To Chalk 416

Chalk - - - - 76

Total - - 492

ECONOMICS. 143

WiMBLBDON.

Shaft 200 teet, the rest bored.

Sand and gravel - - -

Feet
- 10

London Clay. Blue clay, sandy at bottom

- 431

Beading Beds. Mottled clays and sands -

•- 74

Thanet Beds. Dark clays and greenish sands

- 22

To Chalk -

- 537

Chalk - - ' - - - 30

Total - - 567

In the estimation of the available water-supply to
any locality, from deep-seated springs, the formations
beneath it are surveyed in the manner described in
Part II. j a section is then drawn from the dip of the
beds, ascertained from observation or from three or
more points of outcrop, all probability of change in
dip or permeability, of fracture and so on, forming part
of the investigation. The probable depth to the water
and the height to which it will rise can then be readily
estimated, due allowance being made for . the varying
conditions, described in Cases 1, 2, and 3, page 132, in
the resulting water-level. It is evident that the details
required for this purpose are obtained, not at the spot
where the boring is to be made, but upon the outcrop
of the beds beneath, and frequently at a great distance
from the point in question.

It must be borne in mind that the Drift deposits
exert an appreciable influence upon the amount of
water within the beds upon which they lie. A cover-
ing of Boulder Clay must exclude a good deal of water

144 ENGINEERING GEOLOGY.

from an outcrop in some cases ; in others it may even
add to the amount received by those which are so situ-
ated as to catch what runs ofE from its surface. In
some localities the Drift gravels yield large quantities
of water ; if they occur in any extent and beneath the
Boulder Clay, it may be of good quality ; but when
they form the surface deposits, care must be taken to
ascertain that their springs are not polluted by drains
or other sources of surface contamination.

CHAPTER V.

SPEINQS AND WATER-SUPPLY — continued.

Artesian Wells— Absorption Wells.

Artesian Wells, — These wells take their name from
Artois in France, where first used in western Europe,
although they were employed in the East in very early
times. They are borings rather than wells, and in
their simplest form are only holes made down to deep-
seated springs, from which the water rises in those
springs to its normal level. The making of the bore-
holes and the machinery employed are matters of pure
engineering ; but there are many problems in connec-
tion with such enterprises which demand prior geolo-
gical investigation. These are, briefly, the thickness
and hardness of the strata which must be passed
through to reach a particular spring, the number of
springs that may occur within a given depth, and the
height to which the water from each will ascend.

Upon the hardness and thickness of the beds, and
especially upon the frequency of variation in those
particulars, must depend the cost of the undertaking,
for it naturally makes a great difference whether a
boring be made in a given thickness of rock of similar
character throughout, or in the same thickness of rock

146 ENGINEERING GEOLOGY.

in which many refractory bands occur. From this
variation and other causes the bore may have to be
repeatedly constricted as it descends ; it has indeed
often happened that a bore-hole, commenced with a too
small diameter, has been carried a greater part of the
way down to a spring, and then been of necessity
abandoned. It is but seldom, although it is sometimes
the case, that Artesian wells are commenced on abso-
lute speculation, or without some idea of the source of
the springs whence their supply of water is to be
derived.

If water from a certain series of beds be desired, the
depth to them, and their local natnre, may be ascer-
tained by accurately surveying their outcrop ; of course
when there are other borings or wells in the imme-
diate vicinity, particulars of these will facilitate the
inquiry — the outcrop may be near or distant, but there
and there only, in the absence of other wells, is the
requisite information to be gained. The main points
to be ascertained, in the manner described in Part II.,
are the height of the outcrop in relation to the spot
where the boring is to be made, the width occupied by
the beds at the surface, their permeability, the angle
of their dip, and its direction. Prom these data, if
correctly observed, can be calculated the depth at
which the beds will be met with in the boring, the
height to which the water will ascend, and, within
certain limits, its quantity and quality. An estimate
can then be made of the cost, which must be based
upon the hardness of the beds, their variation, and the
depth of the spring below the scene of operations.
There may be, as we have seen, several beds or

ECONOMICS. 147

series of beds, each producing springs of similar or of
different character; these will be at various depths,
and their waters may stand at different levels, but the
method of calculation applies to all alike, and that
spring which is most suitable can be selected. In
passing through the upper beds yielding springs, pre-
cautions will of course be taken to exclude their waters
from the bore-hole where that from a lower spring is,
for any reason, desired. If the water from a lower
spring be estimated to stand at a higher level than that
from an upper one, and the latter be not excluded, the
water — assuming the yield in each case to be equal— ^
would not reach its own levoJ, but would be absorbed
by and would remain at that of the upper spring. By
the same rule, if a boring be continued, after reaching
a spring, until it pierces beds yielding water having a
lower water-level than that of the spring first tapped,
the water from the upper spring will stand at the level
of that of the lower. Or, the lower beds may be per-
meable but yield no water, and the conditions affecting
them such that if they did yield water it would stand
at a certain level ; then the water from above would
be entirely lost in them until they were saturated, if
such were possible, up to that level, throughout their
entire extent.

* In forming plans for an artesian well, the first point is to
determine exactly the size and depth necessary to obtain the
requisite number of gallons of water per day, or rather the
maximum number of gallons at any portion of the day in order
that a reserve may be provided if that quantity exceeds the
probable yield of the spring. Provision may be made for this
by having a shaft of sufficient diameter and depth below the
water-level to contain a quantity of water above and beyond

10—2

148 ENGINEERING GEOLOGY.

-■■--- — -

"what rises from the spring, equal to a difTerence between it
and the demand during the period of largest draught. This
reserve being exhausted, or nearly so during that period will
be replenished in the intervening periods, when pumping is
carried on at a rate less than that of the flow of the spring,
or when it ceases altogether. Should a very large reserve be
required, headings or chambers may be driven horizontally
from the shaft, and these will increase to any extent the space
available for accumulation. In certain cases such headings
driven in the direction from which water may be found weep-
ing into the well will increase the supply, but care must be
taken that the additional water thus obtained is of a quality to
render its use desirable. If not pure, it must be excluded by
cylinders or other means, and the chambers will then be driven
in a different direction, or in some stratum which is quite im-
permeable.

*It will next be necessary to decide whether an ordinary
well dug down to the spring is to be preferred, or a shaft for a
portion only of the distance supplemented by a boring. The
decision will depend entirely upon the depth to the water-
bearing beds and the level at which the water will stand, the
circumstances which ma^ influence the necessity for a reserve
being at the same time taken into consideration. If a boring
be decided on the shaft should be carried down, where practi-
cable, several feet below the water-level, even if the pumps be
not fixed below it, as the supply, when drawn from a body of
water in a well, is less likely to be turbid than when taken
direct from a boring having a much smaller diameter.

' In some cases it is desirable to put down a small trial-bore ;
for instance, when the indications of depth or water-level are
exceptionally obscure, or where they are known to vary rapidly
within a small area. The smaller hole is made at much less
cost than a serviceable bore, and sometimes it may prevent a
fruitless larger expenditure, but the necessity for such trials
forms the exception. There is, however, one set of conditions
in which their use is recommended — where springs are known
to exist with a possibility of their waters being salt from
Laving passed through beds of Tock-salt. Such beds are

ECONOMICS. 149

frequently local and of small extent, affecting certain springs
only, those at a contiguous spot holding no saline ingredient in
solution.

* The kind of material best suited to the purpose of lining the
shaft is governed entirely by the nature of the rocks through
which the well passes. Some strata are coherent and will
stand like a wall for any length of time ; some, although solid,
are very liable to cave in ; others are of a crumbling texture
and must be supported, even during the execution of the
work— the solidity and stability of all, whether hard or soft
rocks, may be affected by the presence of water within them.
Each case must, therefore, be ruled by the details of the strata
through which the well is to be made — in some, iron cylinders
will be required for the whole depth of the shaft ; in others,
for a part only will be sufficient. In rocks which are fairly
dry and firm bricks may be used for lining the well with or
without a coating of cement inside, and in others (although
these form the exception) no lining whatever is actually
necessary.

' The expense of sinking or boring for water is not always
proportinate to the hardness of the rocks to be penetrated.
Frequently those which are the most compact in a hand-
specimen, are the most readily pierced, owing to their being
jointed in several directions ; whilst, on the other hand, some
sands which may be ground into dust between the fingers, are
so tough and coherent in the mass that they have to be picked
to pieces with chisel and hammer, or even blasted with gun-
powder. Sometimes sands full of water, termed " quicksands,"
are met with ; these demand special precautions and perhaps
considerable outlay, if it be necessary to pass through them and
to exclude their water.

' There are cases in which tube wells may be advantageously
employed at a cost much below that of artesian or even of
ordinary wells. Although chiefly adapted and generally used
for obtaining a supply from surface springs, they have been
successful in tapping those at .a considerable depth. In a loose
material, such as saiid, the pumping from a tube-well forms
and gradually enlarges a cavity around the base of the pipe

150 ENGINEERING GEOLOGY.

which acts as a small underground reservoir. Assuming the
cost of the tube-well pipes to be the same as of those used for
lining a bore-hole, any saving arises from the difference
between the cost of boring and that of driving, and under cer-
tain conditions this is material ; but a tube-well can be suc-
cessful only where the strata are throughout such as will admit
of the pipes being driven through them.

'A plan has been highly recommended which may be de-
scribed as intermediate in character, combining the peculiari-
ties of the tube and artesian well, applicable wherever water is
to be obtained, and independent of the hardness of the rocks.
It is to attach a pump to the pipes lining either a tube-well or
a bore-hole, at a certain distance above the water-level, and
without a shaft for the accumulation of water. By construct-
ing an air-tight chamber between the pump and the surface of
the water, it is affirmed that the supply from a low spring
may be considerably increased, whilst the water is thereby
freed, at the same time, from much of the matter it may hold
in suspension.

* All wells and borings, of whatever kind, should be carried
down to the springs which yield the most copious supply of
good water ; when, however, two or more springs exist at a
workable depth, and there is no appreciable difference between
them in regard to yield or quality, that one should be chosen
from which the water will stand at the highest level. This is
a very important point, upon which depend the depth, size and
cost of the shaft in ordinary and artesian wells, and, in a great
measure, the cost of the pumps and pumping the water to or
above the surface. The distance down to the spring, not to the
water-level^must also determine, in connection with the nature
of the rocks, the size of a bore-hole at its commencement. A
bore may continue to a great depth unaltered in size where no
change occurs in the character of the strata, but when soft
rocks alternate with those which are hard it has to be
frequently constricted ; therefore, unless a bore-hole be begun
of sufficiently large size, it may, of necessity, be decreased to a
size at which it cannot be continued down to the point it was
intended to reach. The bore should be at starting of a suffi-

ECONOMICS. 151

dent diameter for this reason also ; if it has to be contracted
many times owing to, hard rocks or other impediments,
although it may eventually reach the spring, it will otherwise
be too small to yield water at the rate which may be necessary
even if the spring itself is capable of affording the requisite
supply.

* There are several ready methods of testing the yield of a
spring, one being the delivery into tanks of so many gallons
of water in a given time from pumps capable of lowering the
water-level. Another is to note the rate at which the water
fills the shaft after having been pumped down, the calculation
resting on the size of the shaft and the number of seconds or
minutes it may take for the water to regain its normal level in
the well.**

Absorption Wells, — The phenomenon mentioned on
page 147, of one spring absorbing the water from
another suggests a valuable, although not well-known,
practical application. It has been stated that strata
which now throw out springs would, if occurring at a
different level, or if inclined at a suitable angle, be-
come the means of draining water away from the
surface. This would occur as a matter of course, with
reversed conditions, for the water would merely be
passing through the same beds in another direction ;
the springs of one locality* are in fact but the natural
drainage of another.

The proposition might not at first be readily accepted,
but a mementos thought convinces of its truth, that a
well which is capable of yielding a given quantity of

* Extracted from an article on ^ Pure Water and its Sources,'
by the Author, in The Brewers' Journal, 1879, in which are
given simple directions for the qualitative analysis, and for the
determination of the hardness, of natural waters.

152 ENGINEERING GEOLOGY.

water is equally capable of getting rid of the same
quantity by absorption. Water is not elastic like a
gas, and wben it bursts up from a penetrated rock
does so not from the force of expansion, but simply
from that of hydrostatic pressure. And it rises to a
certain height only, that of its normal water-level;
when this is attained in the well, and there is no outlet
below that level, no more water rises. Of course as it
is pumped out more takes its place, if the pumping
does not exceed the yield of the spring, but it never
stands higher than a definite point. Therefore, if
water be put into the well, for the moment tending to
increase the height above that point, the pressure is
reversed, and the water so put in sinks down to the
water level, at once if its quantity in this case also do
not exceed the yield of the spring.

The rapidity with which water flows from a spring
into a well depends directly upon the permeability of
the strata through which it has passed, and upon the
same conditions depends also the rapidity with which
it can flow away from it ; consequently the spring is
capable of yielding and of absorbing the same quantity
of water. This power of absorption has been unwit-
tingly made use of in thousands of instances, and in
some with most disastrous results; the pollution of
surface springs by drains and cesspools needs only to
be mentioned. In this way a very valuable source of
water supply for domestic purposes has been utterly
destroyed in the gravels upon which almost all the old
towns and villages are built, and traces of sewage are
recognisable even in many deep-seated springs. There
are, however, cases in which absorption wells may be

ECONOMICS. 153

employed with safety and advantage, for instance in
getting rid of a troublesome excess of water from
surface gravels, and, on a give-and-take principle, in
connection with water-works and reservoirs ; but as
there are also cases in which they may be made, as
they have been made, sources of injury, if not even of
danger, to the community, their use should certainly
never be permitted, except under official sanction and
supervision.

CHAPTER VI.

BUILD INa SITES.

The bearing of geology upon the important problems
of the causes by which health and disease are influenced
is briefly referred to in page 52 ; and the discpvery of
those causes, with the means of their extension or
amelioration, forms the basis of many engineering
designs and operations. Sanitary engineering is a
modem profession, evolved from the study of such
causes, almost during the present generation, and many
of the larger works of the present day are designed
with reference, not merely to the necessities, but, also,
to the health of the community. Such are schemes for
the effectual drainage of large towns, the disposal of
sewage, and the supply of pure water in quantity suf-
ficient for the requirements of the population. These
miist all be determined more or less by the physical
and geological features of the districts where the works
are required. The methods employed in the surveying
and proper interpretation of the phenomena presented
in the geology of a country have been described in
the preceding pages ; a few remarks and suggestions
f oUow upon those which are of a purely physical nature,
or rather, those which, varying as the geology, may
nevertheless be described as surface configuration.

ECONOMICS. 1 55

A most important matter, and one whioli merits
more attention than it usually receives is tlie selection
of sites for Public Buildings and for private Residences.
On small estates there may not be much choice of
situation, but it rarely happens that the limits are so
narrow, or the spot so arbitrarily marked out, that
nothing can be said on the matter. And even if the
exact locality be determined by necessity, or by con-
venience, there still remains for discussion the
questions of aspect, shelter, scenery, and so on.
Where there is ample room, and no reason why one
spot should be preferred to another (except the desire
to select the best possible site) the physical geography
is the chief, if not the sole, consideration. For this
embraces the distribution of land and water, the
geological structure, and the climate of the district ;
these have a definite relation to each other, and upon
them depend all the phenomena by an acquaintance
with which a choice should be influenced.

Much of the beauty of a building depends upon the
site on which it is placed — that is, upon the fitness and
harmony of its immediate surroundings. That which
in one spot would strike the beholder as the very type
of artistic construction, in another and different locality
may seem exactly the reverse. And this is owing, not
so much to the kind of material employed, or even to
the style of achitecture adopted, as to the planning of
the edifice and grounds in regard to their situation.

The term ' site ^ may have either a restricted or a
comprehensive meaning : in the former impljring a
particular spot in reference to a small area, such as in
a plot of ground or on an estate ; in the latter indica-

156 ENGINEERING GEOLOGY.

ting some portion of a large district or physical for-
mation. For example, the English National Gallery
stands on what has been termed ^ the finest site in
Europe,' referring to its position in regard to proxi-
mate surroundings, on a gentle eleration with com-
manding approaches from every direction. This is
but a small part of the ' site ' of the metropolis itself,
which in the midst of a broad flat valley cannot, when
viewed on a large scale, be considered ' fine,' notwith-
standing its many undoubted advantages.

In the selection of a site in a limited area the
position on the map or plan would first be approxi-
mately determined with regard to existing roads,
drainage, and water-supply, and finally settled with a
view to ultimate appearance or elevation. The roads
will vary in nearly every instance, and the selection
can be subject to no special rules regarding them,
but it may be remarked that an important building
should never be placed near a road simply for the sake
of convenience. The additional privacy to be obtained
by placing it at a little distance from the thorough-
fare, and the extra facility thus afforded for surround-
ing it with lawns or shrubberies, are surely worth
some little sacrifice on that score, to lay no stress on
the consequent improvement in appearance.

The drainage, by which term is meant the natural
flow of water, presents a twofold aspect ; not only must
the getting rid of waste and rain water from a spot be
considered, but also the treatment of that which will
find its way to it by gravitation. It is not merely by
running from a higher to a lower level on the surface
of the ground that water abounds in some places more

ECONOMICS. 157

than in others ; it is by constant underground percola-
tion that many situations are rendered damp, where
dampness would never be suspected from the surface
conformation. Questions of site in respect to artificial
drainage are at once solved by difference of level — ^it
must simply be somewhat above the point of discharge,
whether it be from a main drain, tank, or watercourse ;
but the natural drainage towards any spot involves a
consideration of the geological structure. There may
be a water-bearing stratum, a few feet only in thickness,
the presence of which cannot be readily detected with-
out special examination. If a house or other building
be placed on the outcrop of such a bed the result is
perhaps permanent dampness and discomfort therein;
if it be but a short distance above or below, the house
may be perfectly dry, and the water still made available.

But it is chiefly in regard to water supply that the
geological phenomena should be taken into consider-
ation; in this respect also the edifice should be so
placed in regard to the water-levels previously de-
scribed, that pumping may if possible be unnecessary,
or where at all events a good portion of the supply
may be secured by gravitation.

All the points of plan having been considered, the
question of elevation arises, and how this may be
effected by the selected position ; it may be found that
a spot somewhat more to one side or the other will be
preferable in this respect, yet possessing the same
position relative to certain roads, water supply, and
heights for drainaige.

Where it can be avoided, houses should never be
placed just at the brow of a hill, but so far back from it

158 ENGINEERING GEOLOGY.

that the view from the windows does not command all
the ground beisween it; and the valley below. Some
portion, however slight, should be lost, so that the
* foreground ' and ^ middle distance ^ may be definite
and distinct, not merged imperceptibly the one into
the other ; this point is well worthy of remembrance.
Any water, and especially ornamental water, ought to
be visible from the windows ; where nothing of the
kind exists, it may frequently, and with little trouble,
be obtained. Not by scooping lakes or ponds, but
by damming a water-course so that it shall partly fill
its valley ; this is thought by some not to constitute
ornamental water, but the idea seems to be erroneous,
for water in a valley looks more natural than on hills
and slopes, and there is consequently a fitness about
it which adds to, rather than detracts from, real
beauty.

For the selection of a site, in its largest sense, that
is, a spot in some extended district best suited to the
purposes, many points have to be taken into con-
sideration which constitute the details of the district's
physical geography. The height above the sea of
any spot is, for many reasons, an important element,
and one which generally does not receive its due share
of attention. The elevation of most places may be
readily ascertained with a fair approach to accuracy,
for the heights of many points, such as churches,
cross-roads, and so on, are figured on the more recent
ordnance maps, and the heights of contiguous spots
may be estimated by comparison. As a rule, the
higher the spot the lower its mean annual tempera-
ture, but it does not therefore follow that houses

ECONOMICS. 159

standing on high ground must be cold, or those warm
which are built on the lowlands. It is more frequently
the reverse, for so many other influences affect the
result, although those influences themselves are again
partly dependent on the height above the sea. These
are, the form of the ground, the direction of the slope
(if any), the rainfall, and the dampness of the soil, all
partly due to the geological structure. The form of
the ground may be either hill, valley, slope, or table-
land ; if the latter, the spot will probably be warm
and dry. A slope should incline towards, or nearly
towards, the south; it then receives a full share of
sunlight and warmth, and is sheltered by the rising
ground in the rear.

A site in a valley may still be at a considerable
elevation; such sites are liable perhaps to sudden
floods, but dry for a greater part of the year ; houses
in such situations are well sheltered, unless the direc-
tion of the valley meets that of the prevailing wind,
and are perhaps as homelike as any that can be found.
In the low-lying areas such as broad, flat valleys, a
great deal of water is for a time retained, not only the
rain which has fallen within them, but that from the
high lands in addition, and they are exposed to the full
sweep of the wind. For all these reasons an elevated
site is much to be preferred to a low one, in all those
cases where health, comfort, and appearance can
receive their due share of consideration.

The annual rainfall in Great Britain varies from
20 to above 50 inches in depth, its maximum occur-
ring on the west coasts, whence it decreases in an
easterly direction, the minimum fall being in Lincoln-

160 ENGINEERING GEOLOGY.

shire. The rainfall of any intermediate spot is not
strictly proportionate to its position between these two
points, as local causes produce very great variation.
These causes are proximity to, and distance from, the
sea, the directions of the principal valleys, and above
all others, the height of the hills and mountains. But
the rainfall of a district is really no criterion by which
we can estimate the climate, unless it be considered in
connection with other phenomena. A place may have
a very heavy annual rainfall and yet be comparatively
dry, or a light one and be nevertheless damp during a
great part of the year. The rain may be, as in lower
latitudes, very heavy for a short time, a great many
inches falling in a few hours ; but if the physical con-
figuration be suitable, the water runs quickly away to
lower levels and makes but slight local impression. Or
it may be that the soils, and the strata beneath them,
are highly absorbent, such as chalk and sandy forma-
tions, into which the water passes nearly as quickly as
it descends. On the other hand, a locality, especially
if it be low-lying, may be subject to almost constant
drizzling rain, small in amount but persistent, and this
produces a damp atmosphere. In respect to the rain-
fall, therefore, as well as to the form of the ground,
those spots are to be preferred as sites which have a
considerable elevation; the actual quantity of rain
there will probably be greater, but the water will not
remain, and the chances of an occasional drought are
better than those of perennial saturation.

INDEX.

Absorption wells, 161
Agriculture, 35
Alabaster, 123
Alluvium, 104
Alum, 121
Angles of repose, 22, 25

, table of, 23

Annual rainfall, 159
Artesian wells, 145
Aqueous rocks, 10, 12

Bagshot Beds, 108
Ballast, 29, 104
Banks, 25, 47
Bath brick, 105

Oolite, 119

Blowpipe, 57
Bone beds, 107
Books of reference, 57, 137
• Boulder clay, 106, 143
Boundary lines, 64, 66 .
Bovey coal, 107
Brickearth, 29, 33, 105, 106
Bridges, 26, 50
Building materials, 28, 99

, durability

of, 102

102

', selection of.

- sites, 154
Bunter series, 123
Burr-stone, 116

Calcaeeous grits, 117
Cambrian system, 128
Canals, 48

Carboniferous system, 126
Carstones, 113
Cement stones, 108, 113, 122

Chalk formation, 110, 140

Chert, 112

Clay, 12, 33

Coal measures, 126

Cobalt-ore, 130

Colly weston slate, 119

Conglomerate, 12

Contours, 63

Copper-ore, 123, 127, 130

Coprolites, 107, 111, 112, 113,

117
Coral rag, 117
Combrash, 118, 120
Cornstones, 128
Crag, 107

Cretaceous system, 110
Cuttings, 21, 44, 93

Dampness, 51, 157
Deep-seated springs, 135
Denudation, effects of, 70, 91
Devonian system, 127
Dip, 9, 14, 42, 46, 72

, effect of, 14

, rule for finding, 73

— — , example

of, 76
Disease, 52
Docks, 26, 60
Drainage, 26, 38, 49, 156
Drift deposits, 6, 29, 88, 143

Economics, 99
Elvans, 127
Embankments, 26, 47
Eocene group, 108

Faults, 15, 42
Fire-clay, 33, 126

162

ENGINEERING GEOLOGY.

Fire-stone, 112, 128
Flint implements, 105
Flints. 110
Foundations, 26, 50

Gault^ 112

Geological maps, 6, 9, 16, 55^
63, 140

, list of, 17

sections, 55, 92, 140

-, practical

value of, 93

strata, 8

— surveying, 2, 16, 54,

, example

of, 67, 78
Geology, practical value of

2,4
Glacial deposits, 106
Gneiss, 13
Granite, 12, 29, 30
Gravel, 12, 36, 104
Great Oolite formation, 118
Greensand, Lower, 113

, Upper, 112

Grindstones, 125, 126
Grit 12

Hassock, 113
Holystone, 117

Introduction, 1
Igneous rocks, 10, 12, 104
Inferior Oolite formation, 119
Iron-ore, 113, 114, 115, 117,

119, 120, 121, 123, 126, 127,

130

Kaolin, 127
Kentish ra^, 113
Keuper series, 122
Kimeridge Clay, 117

Land-drainage, 38
Landscape marble, 122
Laurentian system, 130

Lead-ore, 123, 127, 130
Level of the sea, 96
Levels, 95
Lias group, 121
Lignite, 107, 108
Limestone, 12, 13, 29, 31

, hydraulic, 32

— , magnesian, 13,

125

Loam, 12

-, siliceous, 13

Magnesian limestone, 125
Main-drainage, 26, 49
Malm-rock, 112
Manganese-ore, 127
Maps, geological, 6, 9, 16, 55,
63,140

, list of, 17

Marble, 126, 129
Marlstone. 121
Materials, building, 28, 99

, durability

of, 102

selection

of, 102
Metals, 35, 99
Metamorphic rocks, 10, 13
Millstones, 113, 126, 128
Minerals, 35, 99

, fruitless search for, 1

Mining, 43
Miocene group, 107

Natural slopes, table of, 23
New Red Sandstone, 123
Northampton Sand, 119
Notes of sections, 79, 85, 86

Oilstones, 129
Old Red Sandstone, 128
Oolite group, 116
Ores of cobalt, 130

copper, 123, 127, 13C

iron, 113, 114, 115, 117,

119, 120, 121, 123, 126, 127,

130

INDEX.

163

Ores of lead, 123, 127, 130

' manganese, 127

■ silver, 127

tin, 127

zinc, 127, 130

Ornamental water, 158
Outcrop, 66
Overlap, 16, 42
OxforclGay, 118

Peemian system, 125
Phosphates, 107, 111, 112, 113,

117,130
Pipe- clay, 107, 108
Plaster of Paris, 123
Pliocene group, 107
Portland Beds, 116

r stone, 116

Purbeck Beds, 116
■ marble, 116

gUAETZITE, 13
uicksands, 26, 49

Eainfall, annual, 159
Kainwash, 26, 36
Reading Beds, 108
Recent deposits, 104
Reference, books of, 57, 137
Reservoirs, 48
Rhaetic Beds, 122
River-gravels, 27, 104
Rocks, 9

, aqueous, 10, 12

9 arenaceous, 12

, argillaceous, 12

— '—-, bearing of, upon prac-
tical works, 20

, calcareous, 13

, cohesion of^ 24

, determination of, 65

;: ■ , tests

for, 66, 58

, granitic, 12

, Igneous, 10, 12, 104

, metamorphic, 10, 13

, nature of, 10

Rocks, old classification of, 11

, permeable, 137

, relations of, 13, 41

, resistance of, to crush-
ing, 34

, stratified, 10, 31

-, table of weights of, 34

— , trappean, 12
-, unstratified, 10, 30
-, volcanic, 12

Rock-salt, 123

Sand, 12, 36, 106
Sandstone, 12, 29, 32
Saturation, line of, 112
Schist, 13

Sections, filling in, 97
, geological, 92

, practical

value of, 93

, notes of, 79, 86, 86

Sewerage works, 38
Shale, 12
Silurian system, 128
Silver-ore, 127
Sites, 154, 156

, selection of, 156

Slates, ]3, 119, 127, 128, 129

Slips, 45, 48

Slopes, table of natural, 23

Soils, 35

Springs, 3, 38, 40, 49, 61

, deep-seated, 14, 135

, nature of, 131

, surface, 107, 133

Stone, testing, 102
Stonesfield slate, 119
Strata, geological, 8

, table of, 100

Stratified rocks, 10, 31
Strike, 72
Sun-bed, 122

Surface-pollution, 134, 136
Surveying, geological, 2, 16,
54,66

, examples

of, 67, 78

11—2

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